you get “every structure that ever existed here”, basically. you still need generations of grad students and pickaxes to figure out which buildings coexisted in time and use.
the real win is that now you know where to dig, and that’s a huge deal all by itself.
You'll very often see a popular side of town fall out of fashion, the buildings get torn down and now it's just empty lots while another side of town is built up and is now fashionable. In 20 years, the buildings fall into disrepair and since rents are so cheap back on the other side of town and empty lots abound, the other side of town is now rebuilt.
And no, we can't just wait for the market to self-correct for empty properties being devalued due to being surrounded by empty properties: you'll just end up with empty, abandoned buildings that nobody wants to fix.
If you want incentives to make people avoid doing that, the only two methods that I can think of are property tax, or legalised squatting (with squatters having a right to stay).
What up property tax? In my city you get reduced property tax (but not zero) if it's commercial building that is empty and unused, but I can imagine there are cities where demolishing an empty building is more cost effective than paying property tax on it every year.
On the other hand, demolishing it also costs money, and with property tax rates on the order of 1% or less, it will only return in something like a decade.
I was just at a site in Israel which over two thousand years was first a Hellenistic city, then a Crusader castle, and then an Arab village; at each of these steps, there was a translation sideways by about 500 meters (making their footprints non-overlapping).