SO many reasons...but I've thought of a few today.
Without us they'd never be able to find anything...man walks to the edge of a room..quick glance around.."Nope! I don't see it, I guess I'll go buy a new one". Woman walks into the room, picks up one thing...yep, right there.
Without us there would be no babies for obvious reasons, but lets just assume for a minute that men had to be pregnant and birth babies. They'd never make it that far. EVERY pregnancy would automatically be prescribe bed rest, simply because they were tired and didn't feel good. Maternity leave would be 5 years long because there'd be no way they could get their shit together enough to go to work AND deal w/a baby/toddler/preschooler. McDonald's would be overrun w/ dad's and little kids at every meal time, because 'who can watch the baby and cook at the same time'?
What the hell would they do w/ a period? They have no idea that once a month we feel like our uterus is trying to claw it's way out of our bodies like that scene in Alien. Having to walk around w/a mattress between your legs or the equivalent of 14 cotton balls shaped like a dick shoved up your crotch to staunch the flow. Being terrified of a blow out while wearing light colored clothes. Have ZERO..yes I mean ZERO(I know it's really REALLY hard for men to understand) interest in sex, as a matter of fact if you get to close to me you may just lose your favorite(yes...we know it's your favorite) appendage. Uncontrollable urges to eat mass quantities of chocolate, more cramps, feeling like you need a nap all day, every day for a week, no emotional control...it's so fun. If it were up to men, they'd have a one week vacation from life every month.
The shower would never get clean because no one is adult enough to clean it. The same goes for the toilet and well... most of the house. However the CAR(truck..whatever) would be spotless.
Appointments would be missed, payments wouldn't get made, the world would be in a state of half-assedness (great word huh!:D) Life would go on I suppose...for a while.
Maybe you're lucky, maybe you have one of those great guys that I hear about, ya know the elusive 'perfect guy'...the rest of us...well we'll just keep cleaning up after the men of the world, cause no one else will do it.