It does a few things. First of all Russia seems to lack military assets in the region, so Ukraine can advance rather easily. Russia has to take back that land, otherwise Putin looses a lot of his reputation and it is obviously something to be traded in in peace negotitations. So Russia has to move troops and material from where they have them, which is in the south. Ukraine has been blowing up train engines, so that is not easy and Russia has been loosing tanks, ifv and all sorts of other vehicles for years. They also need more fighting power to push the Ukranians out, which is most likely going to happen. In the meantime Ukraine gets to shoot at unorganized units setting up, take hostages and really make the Russians think about the war being worth it.
If Ukraine pushes far enough, there is a very defensible line further north, which is shorter and mostly protected by rivers. However that is really pushing it. However if I would be Ukraine, now would be a great time to blow up the Kerch bridge. F-16 just arrived and with that probably some intressting new missiles.
Exactly you can use the railing to hang on the outside. Greater speed and much more fun.