[NirAntae]

I'm not sure where these manual map markers would be placed.

In game on some RPCSS map, or in RPCSS?  How would this improve sharing?

In RPCSS.

Essentially it would help facilitate allied teams to work together (or the selling of information, or whatnot).  If, say, Jane Doe and I are allied, and she tells me that her scouts reported an army in hex 17A, on my RPCSS I could put a custom map marker, like in GoogleMaps, and label is "Jane claims army here" or whatnot.  Basically just makes the 'everything we know or strongly suspect right now' easier to visualize without having to break out PaintShop.