Why birth wear?
The hospital has gowns that I can get dirty and not have to take home to wash.
The hospital gowns are stiff, itchy, open back, no access for monitors unless the gown is hiked up above your belly, with little freedom for movement.
I can just wear a sports bra for my water birth.
Have you ever tried to take off a wet sports bra?
I can wear a tank top and underwear at the hospital.
Do you want to be taking your underwear on and off every time you need a vaginal exam. Have you ever had to take off a shirt after birthing a baby, while hooked up to monitors and IV, so that you can do skin to skin?
I'm just going to end up naked anyways.
This may be the case after transition, but for most of your labour, you will likely be clothed. When you are going through transition, do you want to be worrying about trying to get your underwear and pants off, or feeding your bra strap or robe through an IV line? (As a doula, I've done this way too many times)
Birthing naked also means that if you are hiring a birth photographer, none of your photos will be shareable, which is something to consider, since birth photography and videography are becoming so popular.