Why is there a dirt floor under the bathtub?

We're looking at buying a house and during the inspection we found a small access door in a cupboard behind the guest bath. When we opened it we saw the underside of the bathtub and what appears to be a large hole in the slab on grade foundation with several pipes coming up from the dirt floor. I have tried to find why they would leave a hole in the foundation here. My best guess is to better access a any clogs in the S shaped curve of the tub drain (which would be underground) but I don't know for sure. Can anyone tell me why there is a hole here, if it is common practice, and if there are any potential problems of having this?