For propagating plants from seed, we (the Salish Seeds Project) top most of our small pots/cone-tainers with fine chicken grit. It does keep small seed from washing out, particularly if we accidentally blast the pots with a strong spray of water. It also acts as mulch for conserving moisture. The containers don’t dry out as quickly when topped with grit. I have also read and been told that a thick enough layer can deter the growth of mosses and fungal disease. Coarse sand, which is less expensive than grit, can also be used. You can certainly germinate seeds without a gritty topper, but it’s a nice trick to make the process a little easier and sometimes more successful.