Other than making starters, I've always brewed with someone else. There's a lot to do...someone can watch the pot for hot break while the other person gets the hops / next extract addition / oxygen stone ready. There is plenty of cleaning, of course, and hauling carboys down into the basement, notes to take, beers to pour, etc.
It's nice to have help lifting heavy kettles. I have to move the boil pot off the stove over to the sink without getting the chiller lines tangled in my feet; one guy carries the pot while the other one moves the hoses. If you were mashing with a gravity rig, something is probably going to have to get moved (hot water, or a mash tun, or a boil pot). Sure, a person *can* lift 50 lbs themselves, but why risk the hernia if someone else is around?
I've also found bottling to go much more smoothly with two people, one to fill and the other to cap. This will minimize the time the bottles are exposed to air as well.
Having a third person means you get extra help with dishes (one washes, one dries, one puts away) or bottling (one guy fills, one guy caps, one person rinses/dries/stores bottle), or most importantly someone to watch the grill.
I've had two or three people show up as the third guy often enough that they could brew on their own, which is cool if one of the first-stringers can't make it on a given day. We also have a rule that if you want us to make beer for you, you have to show up and you *will* be put to work.
Finally, if you have help you can make more beer at a time. We had three kettles boiling one day (two on the stove, one outside on a turkey fryer) and I'm glad I had a crew of five. It was busy, but we made 15 gallons of beer in the time it normally takes to do five.