Lye certainly isn't about the flavor
Like many spices, salt, and some other things, it's probably about trying to keep the food from rotting on you and killing you. Not sure why lye would occur to the ancients before, say, salt. It's also possible that the function of the lye is softening dried fish, which would otherwise be like eating the sole of a boot (heh).
Oddly, I hear Norwegian-Americans eat a lot more of it than actual Norwegians do.