And that reason is so you can have all the diaper changing essentials WITH YOU when your baby decides to take a prune-induced shit while sitting on your hip as you fill out a STOOPID form to withdraw your son from preschool (for the summer?? are you fucking kidding me?) and during which time has caused the shit to shoot out the leg of your baby's outfit all over your forearm like an exploding ketchup packet and which is also being smeared all over her thighs as you shift and adjust her weight on your hip.
Failure to bring said diaper bag along with you for the five minute car ride to and from the preschool (because, really, what could happen in such a short amount of time???) WILL result in you having to place said baby face down on the front seat of your vehicle while you rummage through the glove compartment in the middle of the parking lot looking for extra Dunkin' Donuts napkins to wipe up baby shit while the child whom you have come to pick up exclaims how D.I.S.G.U.S.T.I.N.G. the poop is and how WRONG it is and you will just beg him to pleeaassee put on your straps and BE QUIET.
(And as you are rummaging through the glove compartment looking for napkins with your baby face down on the front seat, she begins speaking to you in her baby-grunt language as if to ask you, "What the hell, Ma?!?")
(There were but four... FOUR napkins, BTW)
And then in a moment of desperation you will take the VERY LAST Dunkin' Donuts napkin to push any remaining poop back INTO the diaper along with the napkin creating a make-shift dam to hold back any additional oozing of the poop until you can get home and clean up your baby the right way. With WIPES and a fresh CLEAN diaper for God's sake!