It's important for a) those who are likely to go in a signing round and want to position themselves better relative to other top prospects, b) those who play in conferences that don't get as much scouting attention and want to get on more radars by performing well in a top summer league, and c) those who, because of injury or other lack of opportunity, haven't had sufficient opportunity to demonstrate their skills in college competition.
If a player chooses to work, rest, condition, study, or accept an internship instead of playing in the summer, he is least likely to affect his draft status if a) his summer opportunity is in a middle-ish as opposed to a top collegiate summer league, b) he has already had regular playing time in a mid-major or better conference, and c) he isn't expecting to be drafted early.
The other consideration, of course, is what summer activity is most likely to help the player get better: reps, rehab, or conditioning. Demonstrate pro potential the spring immediately before the draft and no one will care what you did the summer before.