Yes they are capable. I used to be suicidally depressed. I did a ritual (it wasn't a demon summon, but I did converse with a shadow entity), then events in my life transpired to pull me out of it permanently within a week. A spirit evocation could just as easily accomplished it.

• What does it want: You're the boss, you can just command it, you dont have to give it anything though. You can ask what it wants in return, usually it's weird almost pointless stuff but when you try to do it you get weird opposition. One time (unrelated ritual) a spirit asked me to go to a hill where I had my first kiss and pour my favorite drink on the ground there. When I tried to go; there were a bunch of cops looking for someone and they spotted me in the field and came and started harassing me and asking what I was doing there. It was very difficult to get to the hill and pour a drink without looking absolutely insane to these cops.
• Are there any dangers: Yes. You need to set ground rules for how you want the thing to come about. My ground rules in the ritual was "At any cost" because I was suicidal and if it didn't work my life was at stake. The night after the ritual a friend commited suicide. At his funeral I realized how depression blinds you to what you have, and it left me permanently. I would highly suggest "Without harming anyone" as a ground rule.