I am adding this answer because after seeing the question Is there an expression to describe a situation where someone could have helped you but they didn't? I came away with some confusion over the difference between expression-requests and phrase-requests and questions in my mind whether they were the same--and found this meta question & answer, which addresses some of these same issues--but the suggestions to merge the two tags hasn't been addressed yet publicly from TPTA, the powers that are.
Because when I clicked on the expressions it says
This tag is for questions about expressions. Expressions are words or phrases used to convey an idea, or else a particular term used conventionally to express something. Consider phrase-requests if you are looking for an expression, phrase-meaning if you are unsure about the usage of a given phrase....
but when I clicked on phrase-requests it says
This tag is for questions seeking a phrase that fits a meaning. If you're specifically seeking only a single word, see the "single word requests" tag too.
and I thought to myself "Isn't an expression the same thing as a phrase that fits a meaning," so I went to expression-requests and found no definition of the tag, which was not helpful. So, to answer your question title, I don't know what it's for.
I looked at the number of questions under phrase-requests and expression-requests and they both appeared substantial and the questions with these tags seem to be asking the same thing.
So my thought was to come here and ask what the difference was between the two tags (as well as point out that expressions-requests has no definition. But the software suggested this question here that I am responding to as a possible duplicate, so instead of asking a new question, I'm chiming in with the suggestion that the two tags be merged or somehow differentiated, and in the meanwhile point out that the one tag has no definition. But I don't know if TPTA intend to do anything.