It's a social event, which means guests are supposed to socialize. Since the best man probably does know several of the other guests, it's his duty to introduce his date to some of them so that she isn't stranded with strangers while he's doing wedding stuff.

Where does the best man's date sit?

Traditionally, the newlyweds sit in the middle of the table, with the bride seated to the groom's right. Same-sex couples can feel free to seat themselves as they'd like. For a male/female pattern around the table, seat the best man next to the bride and the maid of honor next to the groom.

What is a best man supposed to do?

The best man is often in charge of the groomsmen, as he plans the bachelor party, helps them dress for the wedding, and even organizes their transportation to the ceremony. He is also responsible for lending a hand during pre-wedding events and the reception. Above all, he is there to support the groom.