Some may not care about texel density, but I think it's important to get "close enough" to what the size should be on a face when you first apply a texture.
If you were making a minecraft-like game with 16x16 textures, you'd set your texel density in Scythe to be 16 pixels per 1 meter and it'll all just work based off of that. A lot of things will rely on texel density in the background, so it's important to at least put some thought into it. You just have to think about how many pixels you want to cover 1 meter, and it'll do its thing.
It'll default to 512px per meter.
Some more videos of it in action: