Advice for outside wall in a room.

Hi there. I've just removed some skirting boards in a room and found something that's puzzling me.

On all of the walls, whether it be an exterior/brick wall or plaster board, there has been a gap of about 10cm left where the wall doesn't touch the floor. Is there a specific reason why this has been done? I'm planning on making the gap smaller as my new skirting boards aren't as high as the old ones and they need something to adhere to. Is this ok practice.

Finally, on the brick wall, what do I use to fill the gap? I'm assuming it's not plaster board.

I think this video will explain things better regarding the bricked wall.

Thanks

It’s normal to keep the plaster off the floor a bit, to stop damp bridging from floor into plaster (although arguably it shouldn’t anyway), but 100mm is a bit much. Yes it’s ok to fill the gap. Use any kind of plaster you’ve got knocking around, ideally bonding/hardwall type backing plaster then cut the surface back by a couple of mm and skim with multifinish. Or you can get one coat plasters.