Some context: I recently did a job as a proofreader for a translated document. This quickly turned into editing, which in turn turned into mostly re-translating. That's what most of this based on.
It is very useful for an editor - and even a proofreader - to know the source language.
For a proofreader who is literally just looking for typos and the like - obvious mistakes in the target language - this is less important. Someone who is doing that only needs to make sure that what they have in front of them is well-written in the target language, and that doesn't require knowledge of what the original author intended.
But it can still be useful, especially in cases where a single letter can change the meaning (in the target language). Take this as an example:
The painting was a flame.
This could be a typo / autocorrect error for something entirely different, namely:
The painting was aflame.
These mean entirely different things - one is describing what the painting is of, the other is informing you that it's on fire - but there's only a single letter difference. This is a situation where it would be useful for a proofreader to have the original text and to understand it.
For an editor, it's almost essential for them to be able to understand the original text.
An editor needs to rewrite things. To make sure that they're staying true to what the original author intended, they need to know what they wrote. What they have right now is the translator's interpretation. This is certainly useful, but sometimes, you need to be able to look at the original text.
An example would be idioms. I've seen translators translate idioms literally. An editor's job would then be to replace this idiom with an idiom in the target language. For the editor to be able to do that, they need to know the language and to know that this is an idiom and what it means. While it's technically possible to do this without speaking the language - the Internet is your friend - it's much, much, easier when you speak the language.
Also, translators aren't infallible. Especially if the author isn't that familiar with the target language, they can't always read through it to make sure that it's accurate. In a job I did recently - ostensibly editing, but eventually ended up almost re-translating it - the translator didn't do a great job. They relied heavily on Google Translate, and the translation wasn't always correct. In this case, they had translated it to be the exact opposite of what it was in the original text. The original text said something about it being the first time they drew something not from real life or remembered; the translator wrote down that it was the first time they had drawn something from real life / memory.
Your editor has to be able to recognize when the translator has made a mistake (in those hopefully rare cases). Getting a second pair of eyes on things is definitely better.
Also, something to keep in mind: The editor and the author, if the editor doesn't speak the source language, may have some trouble communicating. If the author doesn't speak the target language (which, presumably, they don't well, or they wouldn't have had to use a translator), and the editor doesn't speak the source language, how are they supposed to communicate, to make sure that the editor is sticking with the author's intent?
It's much better, easier, and more likely to lead to accurate results when the editor speaks the language that the work was originally written in.