TL;DR: If you don’t like Hugo’s prose, why are you reading Les Misérables?!
The lengthy disclaimer is just one paragraph long, a drop in the ocean of this novel, so I will take the liberty of quoting it in full, in the 1887 translation of Isabel Hapgood:
The author of this book, who regrets the necessity of mentioning himself, has been absent from Paris for many years. Paris has been transformed since he quitted it. A new city has arisen, which is, after a fashion, unknown to him. There is no need for him to say that he loves Paris: Paris is his mind’s natal city. In consequence of demolitions and reconstructions, the Paris of his youth, that Paris which he bore away religiously in his memory, is now a Paris of days gone by. He must be permitted to speak of that Paris as though it still existed. It is possible that when the author conducts his readers to a spot and says, “In such a street there stands such and such a house,” neither street nor house will any longer exist in that locality. Readers may verify the facts if they care to take the trouble. For his own part, he is unacquainted with the new Paris, and he writes with the old Paris before his eyes in an illusion which is precious to him. It is a delight to him to dream that there still lingers behind him something of that which he beheld when he was in his own country, and that all has not vanished. So long as you go and come in your native land, you imagine that those streets are a matter of indifference to you; that those windows, those roofs, and those doors are nothing to you; that those walls are strangers to you; that those trees are merely the first encountered haphazard; that those houses, which you do not enter, are useless to you; that the pavements which you tread are merely stones. Later on, when you are no longer there, you perceive that the streets are dear to you; that you miss those roofs, those doors; and that those walls are necessary to you, those trees are well beloved by you; that you entered those houses which you never entered, every day, and that you have left a part of your heart, of your blood, of your soul, in those pavements. All those places which you no longer behold, which you may never behold again, perchance, and whose memory you have cherished, take on a melancholy charm, recur to your mind with the melancholy of an apparition, make the holy land visible to you, and are, so to speak, the very form of France, and you love them; and you call them up as they are, as they were, and you persist in this, and you will submit to no change: for you are attached to the figure of your fatherland as to the face of your mother.
People often refer to Hugo’s novelistic essays as ‘digressions’, but is this really fair? Later in the novel, Hugo writes:
Where the subject is not lost sight of, there is no digression
So let’s call them ‘parentheses’ instead. This particular parenthesis has multiple effects, of which I will limit myself to five.
First, it is delightful to read. The prose is almost poetic in places, and there are striking turns of phrase such as the paradox “that you entered those houses which you never entered”.
Second, it controls the pace of the story. The parenthesis appears at the start of chapter I of book V of volume II. At the end of book IV Valjean suspected that Javert was close to tracking him down at his lodgings at the Gorbeau house, and took flight with Cosette. It is a tense moment in the plot: the reader is in suspense as to how close Javert has come to a capture, and whether and how Valjean will escape the pursuit. By placing a parenthesis at this point in the text, Hugo pauses the story and prolongs the suspense, making the reader stop and consider what has happened, turning a tense moment into a cliffhanger.
(This tactic, of inserting a parenthesis at a tense point in the narrative to heighten the dramatic effect, is one that Hugo uses several times in Les Misérables. For example, at the end of book V Valjean has escaped Javert by climbing over the wall of the convent in the Rue Petit-Picpus, but does not yet know whether he and Cosette will be welcome there. This is followed by two lengthy parentheses: book VI covering the history of the Rue Petit-Picpus, and book VII (itself called “Parenthesis”) analyzing the relation of the convent to French society. Only in book VIII does Hugo return to the drama, so for nineteen chapters the reader has been on tenterhooks about the fate of Valjean and Cosette.)
Third, it adds to the verisimilitude of the narrative. By raising the possibility that the descriptions might no longer be correct, Hugo implies, without coming out and saying so, that they are correct with respect to the time in which the novel is set. If we believe that the geography is accurate, then we might find it easier to believe in the events of the narrative.
Fourth, it reminds us of Hugo’s exile. The reason why Hugo’s knowledge of the streets of Paris was not up to date was because he had fled France in 1851 after the coup d’état of Louis Napoléon. Although the emperor promulgated a general amnesty in 1859, Hugo chose not to return as that would have meant subjecting his writings to the censorship of the Second Empire. Les Misérables was published in 1862, while Hugo was living in Guernsey.
Fifth, it’s a touching expression of Hugo’s feelings of homesickness. When he writes:
Later on, when you are no longer there, you perceive that the streets are dear to you; that you miss those roofs, those doors; and that those walls are necessary to you, those trees are well beloved by you; that you entered those houses which you never entered, every day, and that you have left a part of your heart, of your blood, of your soul, in those pavements.
who can doubt that the you of this passage is Hugo himself, sitting in the study in Hauteville House in St Peter Port, looking out over the water to France, and remembering his former home in Paris?