You can roast fat too. In the very early phase of a new character I either roast and eat the bird fat, use it to bait traps (to hopefully convert it into owl meat), or sacrifice it. In the earliest phase I don't have the time to take care of the skin, and in the marginally later phase I can use bark for tanning.
If you kill a seal, however, you'll get a lot of fat left over after the skin is tanned, and yes, you can roast and eat it.