Material on this deck - what, why, and how to cover?

I recently purchased a house that has an upper-floor deck above the front porch, with a shingled roof above that:

The upper floor deck, i.e. the part you can walk out on from the upper floor, has a dark black dusty material applied in sheets:

I thought I was looking at a thick rubber membrane intended to be the deck surface. However, after putting some plastic chairs on it on a hot day, I discovered it’s actually more of a tar-like substance that smears and deforms under pressure, as you can see here from where the chair legs pushed it up into little mounds:

I believe this whole porch/deck structure was modernized/ refinished around 2012.

I have three questions about this:

1. What is the most likely material that I’m looking at on that upper deck?
2. Why would a material like that be used? To my untrained eye, after seeing the damage from the chairs, it seems to be a waterproofing rooftop material like Bitumen or tar or some such. Is it typical to put that down on a deck ppl are supposed to walk on? If so, is the assumption that something else is put on top to protect it?
3. Most important: how should I cover this so I can use this deck? Three thoughts I had were a) heavy outdoor carpeting/ area rug with thick rubber underside to protect it, b) interlocking deck tiles, or c) place strapping perpendicular to the face of the house to allow run off, apply press board over top and paint it. I’m most inclined toward the deck tiles but wonder if the plastic ridges on the underside will damage this roofing material.