Leitz was probably one of the few to ever offer a full frame carrier. With other enlargers one either uses the glass carrier or just files out a spare glassless carrier. Also, some like the look of the filed out carrier when included in the print.

I only use my filed-out ones when actually printing whole-frame or showing the filed edges. Otherwise ,the non-filed one holds the negative flatter. The glass one holds it very flat, and allows full frame printing, but I actually like to show the filed out border. More so since the advent of D*****, because the negative carrier edge is an exclusive byproduct of the analog enlarging process.