You don't need to of course but in many countries there's grants to purchase in the first place and then you can sell excess back to the grid, so it would be rather pointless not to.

My sister got it installed, about €15k worth, of which about a third was covered by grants and she's been selling enough back to the grid that she doesn't pay much bill at all for about 6 months of the year.

Unless she was some kind of off the grid absolutist, why wouldn't she have registered?