I don't know about diversity receptions because I'm not diverse. But the typical benefits of going to receptions are these:
1) Get your name on the list as having attended for when you apply for next summer.
2) Get business cards, so when you can build your contacts and when you apply next summer you can say "I enjoyed my conversation about this person about this."
3) Find out more about the firm and decide whether you like the attorneys you talk to.
4) Practice being professional.
5) Free food and booze.
Callback interviews are mainly for attorneys to see if they like you. So practice having pleasant conversation that still leaves attorneys with a distinctive impression about you, your personality, and your interests. A lot of the people who stand out at receptions do so for the wrong reasons, and a lot of the other people are polite yet "blah" so they're not memorable.
Come with LOTS of questions about the firm, and have a few strategies to politely exit a conversation. Have good judgment about when to exit.
It is sad to have to say this, but do not: drink too much, use your cellphone, walk away without politely excusing yourself, badmouth your classmates, monopolize attorneys who clearly want to speak to other students, call attorneys by their first names without being invited to do so, etc. A lot of students have stunted professional etiquette.