If it can feed back to the grid, it can be a safety issue for electrical workers. Having them know about this stuff before they mess with lines is the safe way to do it.

Yup, the reason is that undocumented solar panels plugged into the Grid could cause seriously dangerous power spikes and throw off calculations on how much Suppliers need to feed into the Grid. So the only panels that don't have to be declared and registered with MaStR are ones on top of cars and ones that are so remote that you couldn't plug them in in any case (but Germany has basically no places remote enough to meet that criteria)