Maybe because some people might only subscribe to tags rather than to an entire stackexchange site?
And is this also dependent on what stackexchange site it is?
The effect of adding relevant tags is to make the question visible to more users, who are then the users who can reply to the question. This doesn't automatically means more answers, as you could ask a question that needs to be understood before to be answered, or requires that somebody tests what you are reporting.
For sure there are more eyes on your question, especially if there are users who are notified for new questions using the tags they are following, or who can get a list of questions using specific tags.
Sure, choosing relevant tags can help draw attention to your questions.
I don't know if it'll necessarily increase the number of responses (I have no data to claim either way), but it certainly can't hurt.
This may have less effect on smaller sites where the question volume is low enough that most people will see all new questions as they come up.