Now Tom, who does not know you, is having a party. (Tom's a whore.) It is 21+. And for some gawdawful reason, you want to go (probably because you like the band that's going to be playing).
Tom is checking IDs. You so do not want to show Tom your ID, because he's creepy and you just know that he's going to show up drunk at 3am some night if you let him get your address.
However, since Brad knows you, Brad can vouch for you, and you get in. (And out as soon as possible, because OMG the strobe lights OW.) And you're safe from 3am visits from Tom, unless you caved and gave him your address anyway when he came schmoozing up to you (which you really didn't have to do).
This adventure in collaborative LJ-related insanity is brought to you by the good folks at #lj_s, especially remark, for the benefit of mayerman.