You should definitely have a specific intention when you invoke anything, but there isn't one universal reason why people summon angels. Some people will ask for knowledge or messages, sometimes protection, guidance in their magic studies, I've even known couples struggling with infertility to invoke angels so they could have kids. There does seem to be more of a consensus about angels than demons that they're a little tricky to work with, or at the very least will ignore you if you try to ask them to do something that isn't their job.
With all of that said, invocation is magic that has a high risk of going awry if you don't know what you're doing, so it really isn't something to play around with on a whim. If you're still this uncertain about how it works and what it's for, I'd recommend spending a lot more time studying up on the practice before attempting it, and getting lots of practice with meditation, cleansing, and basic self-protection first. Don't dive into the deep end before learning how to swim.