One reason I like royalties, the main reason even? They keep on giving.
GURPS Martial Arts came out in 2007. It sold very, very well - enough that I blew right past the advance and into royalties right away. That was lucrative and satisfying.
2007 was 15 years ago. But I still got royalties this past quarter on 50 copies of GURPS Martial Arts sold as Print on Demand through Createspace. That's besides the steady trickle of PDFs it sells month after month after month.
It's been about 16 years since I wrote that book. So much research in 2004 and 2005. Lots and lots of writing in 2006, and playtesting after that. All that work is long done - yet I still keep getting payments for the work.
That is why I like royalties. If my work has lasting value, I get lasting payments. That's so much more of how I like to view my work.