Yes and no.
If you have a wallet, and you just drop it onto the bed of your glowforge, and you want to engrave a logo on to it, then entering the correct focus height is very critical to a good alignment using the bed image in the GFUI (the browser window.) Even with the exact right height your machine could be off by as much as 0.25 inches and still be “in spec.” My machine has mostly been dead on, but other machines are not. I’m told it has steadily been getting better.
This up to a 0.25" alignment issue is why sometimes you want a jig. As long as the art board in your svg file is set to 12x20 inches the laser will always cut in the same exact spot every time no matter what it looks like in the GFUI. So say you’re engraving a 0.25" coaster and want to use .01" card stock for a jig. Your first svg would be a rectangle the size of a coaster at coordinates x,y. You cut out that rectangle from the card stock. Your second svg is what you want to engrave and is at coordinates x,y as well. So you drop your coaster into the card stock cutout and hit print. No matter what you see on the screen (and it can be off quite a bit, making you very nervous) the glowforge will laser in exactly the correct place.
The second reason focus height is important is that it determines the width of the laser beam at the top of the material. If you have something 0.25" tall and you tell the glowforge it is 0.35" tall, the laser will be aimed .1" above the top of your material. It will be in the right spot left to right and top to bottom, but not up and down. Generally, this will result in needing more power or a slower speed to cut through something and a wider strip of material vaporized by the laser. Normally this is bad, but in advanced lasering it may be a desirable effect. If you’re interested search the forums for defocus.