the 'manhole cover' is an inspection cover for inspection chamber for either sewage or rainwater drain, which ordinarily would have a section of open drain, so that should the drain become blocked, drain rods can be inserted to clear the blockage. That's how the four manholes around our house are constructed. (Built 35 years ago). Like this example:
https://www.google.com/search?q=ins...ate=ive&vld=cid:912fb8cf,vid:Ni5NIEF5I5k,st:0
It looks to me as though there's a terra cotta coloured plastic circular cover down there, maybe for that purpose, rather than a section of open drain. The one the water company uses will be much smaller, simply for access to the stop-cock to turn off the water to your house, and for the water meter if you have one.
If the manhole is for sewage, one thing you might like to try is to get someone in the house to flush the toilet while you listen for water flowing. To hear that, you'd need something like a broom handle (if it's long enough), and press it on the cover at the base of the manhole, while you press the other end to your ear. It might sound primitive but it's one of the techniques which water engineers use to hear water flowing through underground pipes to trace leaks.
The bottom line is that it's almost certainly there for a purpose.
Hope that helps.
Nowadays, the inspection chambers are invariably plastic, like this:
https://www.google.com/search?q=ins...ate=ive&vld=cid:d65f94cd,vid:Wl61iDwjFEw,st:0
