My example use case is:
I wish to host an experience-based combat game in my region. Players would need to accept my game's experience in order to receive a temp attached combat meter and weapon. The parcel also has the experience enabled in parcel settings.
I don't want my players to have build rights on the region-sized parcel but I want their experience-enabled temp attach weapon that my experience provided them, to still be able to rez physical projectiles, overriding parcel build permissions since temp attachments become owned by the person they are attached to. The projectiles in that temp attached weapon would also be owned by the person it's attached to.
By limiting build rights through the normal parcel settings, this ensures that unauthorized weaponry cannot be used for collision based combat and disallows players from rezzing other items/barriers/vehicles that would break gameplay. Letting experience enabled scripts to be able to continue to rez their projectiles despite parcel build permission would aid in immersion. I understand that we can accomplish much using llCastRay as a replacement for physical projectiles but it's not as immersive as having an arrow shot into the air in an arc to hit it's target on the other side of a hill.
Parcel build permissions would probably need a third option besides allowing build permission for everyone or group members. The third option would be Experiences.
I believe expanding Experience permissions could solve a lot of use cases besides just combat situations but it would offer a lot of solutions to making game experiences more immersive and feature rich.