Sometimes Shakespeare uses "do" with a verb even though it isn't necessary. For example, in Macbeth, Act One, Scene 2, line 10, the captain states, "As two spent swimmers that do cling together". Why doesn't he say "As two spent swimmers that cling together"? What purpose does the word "do" have?
The word ‘do’ fills what would otherwise be a gap in the rhythm. With ‘do’ you can scan the line as regular iambic pentameter:
x / x / x / x / x / x As two | spent swim- | mers that | do cling | togeth- | er
(The extra syllable at the end of the line is a so-called ‘feminine’ ending.)
Without the word ‘do’ there would be a gap in the rhythm and the line would be hard to scan:
x / x / x / ? / x / x As two | spent swim- | mers that | ? cling | togeth- | er
There are other ways you could try to fix the rhythm, for example:
As two spent swimmers that hold fast together
but ‘hold fast’ does not have quite the same meaning as ‘cling’, and it is less satisfactory from a rhythmic point of view because you’d like to stress ‘hold’ as well as ‘fast’.
This use of ‘do’ is a kind of ‘expletive’ and is ubiqitous in Early Modern English verse wherever a gap in the rhythm needs filling next to a verb. Just a few lines later in Macbeth Shakespeare has:
The multiplying villainies of nature
Do swarm upon him—from the Western Isles
Again, you can imagine rewriting the line to avoid the ‘do’, for example:
The multiplying villainies of nature
Attend upon him—from the Western Isles
but ‘swarm’ makes for a stronger image, the ‘villianies of nature’ being imagined as thousands of stinging or biting insects.
The Oxford English Dictionary has the following to say about this sense of ‘do’:
This construction appears to arise in the 13th century (no certain examples occur in Old English) and becomes especially frequent after 1500, first as a simple periphrastic form without perceptible difference of sense (in which use in south-western English regional dialect it practically takes the place of the simple form of the verb). In standard English from the early 17th century onwards it becomes restricted to contexts where it is functionally parallel to other auxiliaries (perfect, progressive, and modal). Thus simple affirmative with inversion of word order after certain adverbs: ‘So quietly did he come that…’ (like ‘So quietly has he come’). Emphatic: ‘He did drink’, ‘and drink he did’ (like ‘I will go’, ‘and go I will’). Interrogative: ‘Do you hear?’ (like ‘Will you hear?’). Negative: ‘They do not speak’ (like ‘They will not speak,’ ‘They have not spoken’.)
Before this use of ‘do’ became widespread, poets had to use other devices. Geoffrey Chaucer, for example, writing in the 14th century, uses ‘gan’ (meaning ‘began to’) when he has a gap that needs filling, for example, from ‘The Knight’s Tale’:
And with that word Arcite gan espye
This Palamon gan knitte his browes tweye
And to the temple of Diane gan hye
‘espye’ = ‘discover’; ‘tweye’ = ‘two’; ‘hye’ = ‘hasten’
Over-use of expletives like ‘gan’ and ‘do’ eventually led to a reaction against them. Alexander Pope wrote:
I would except against [i.e., object to] all expletives in verse, as do before verbs plural, or even the frequent use of did and does to change the termination of the rhyme; all these being against the usual manner of speech, and mere fillers-up of unnecessary syllables.
Alexander Pope (25 November 1710). Letter to Henry Cromwell. In John Croker (ed.), The Works of Alexander Pope, volume VI, p. 112. London: John Murray.
By ‘change the termination of the rhyme’ Pope means that poets often found it convenient to write, for example, ‘did look’, instead of the more natural ‘looked’, at the end of a line, in order to have a wider choice of rhymes. Pope was not above using this device himself, for example:
For loved by Pallas, Pallas did impart
To him the shipwright’s and the builder’s art.
Alexander Pope (1715–20). The Iliad of Homer, book V, lines 79–80.