I know, I'm going to get some rebuttals but I expect that. Please note, I said ..why I like synthetic stocks...not why you should like them.
First, let me say that I am a hunter and do not necessarily buy guns for "pretty" sake; function being more primary with me. Anyway, here are some reasons I prefer synthetics:
1) They will not warp or distort in normal use.
2) Usually they lend well toward a forward weight-bias, which I generally prefer for a steady hold.
3) Even the least costly ones have some type of grip promoter..checkering, stippling or otherwise grippable surface.
4) Often they lower the price of a comparable gun.
5) If you consider basic black as too mundane, it can be painted...even often.
6) Paint jobs can include a pattern to match conditions under which one expects to hunt
7) They are not necessarily ugly, since beauty is in the eye of the beholder
Below, see 3 of my rimfire which started out with stocks of basic black. Not long ago, the Ruger was "flash painted, while the Marlin was camo.