In the backcountry, we should bury solid human waste (i.e. poop), and we are told to carry out toilet paper. According to this answer on tissues, for some kinds of waste the ecological impact is small, but we should still leave no trace for social reasons.

If I bury my poop and bury (biodegradable) toilet paper along with it, what is the harm done? The ecological impact should be small, and it is unlikely another hiker will see my toilet paper. Then why do we need to pack it out?

Note: I am assuming areas where burying human waste is considered good practice in the first place, thus excluding areas where visitors are required to carry out solid human waste.