If Data-Flow Analysis is able to approximate, what does that mean?
All of the problems we want to solve with static analysis are undecidable.  We have a yes-or-no question ("will this expression be available?", "is this variable live?", etc.).  Ideally we'd like an algorithm that takes as input the program, terminates in a finite amount of time, and outputs the correct answer to the question.  Unfortunately, the problem is undecidable, so there is no such algorithm.

But we're not willing to give up.  So, we accept algorithms that always terminate but sometimes output "I'm not sure".  There are basically two kinds of such algorithm.  One can will either output "Definitely yes" or "I'm not sure (might be yes, might be no)".  The other kind will either output "Definitely no" or "I'm not sure (might be yes, might be no)".

Underapproximation vs overapproximation just refers to the difference between those two kinds of algorithms.

How do you pick one or the other?  That depends on what you're ultimately trying to achieve.  In some situations, it's more useful to know that the answer is "definitely yes".  In others, it's more useful to know that the answer is "definitely no".

Of course, these two notions can be combined, so you can get an algorithm that will output either "Definitely yes", "Definitely no", or "I'm not sure (might be yes, might be no)".  But you can't avoid the possibility of an "I'm not sure" answer; that's an unavoidable consequence of undecidability.