The alcohol, which is a solvent, penetrates the wood better kinda like the aniline dye Killy asked about. There are natural "oils" in most any wood and some more than others (Pine, spruce etc and hardwoods like mesquite)
Killy: I have not seen any UV effect on some of my wood shafte from 20 years ago when I used alcohol as a medium for both RIT and natural dye.
Yep, I still have two arrows from back then. This could, perhaps, be due to the final coat of varnish which I used then. RIT can be made pretty concentrated...
Hope that helps.