You'll notice all planting instructions reference how deep to plant the seed under the soil. If you're only planting one kind of seed you can probably measure once and then eyeball the rest and do just fine, but what about someone who needs to plant many different varieties of seed, and lots of each type?
If I was a terrible millennial I would call this a "garden hack", but since I'm not I'll just tell you about it. In this case we used an old paintbrush but you could really use anything. Just score a line every 1/4" and then all you need to do is just poke little holes in the dirt down to whatever line you need to. Ink will smudge off in the dirt so it's best to use something made of wood you can carve into with a knife or thin blade. We used to just wing it and poke little holes with our fingers but this actually makes it easier and of course much more accurate.