the 9900K seems capable of an extra 1-2 fps here and there *if/when* using a 2080 Ti and playing games at 1080P, so, many consider it hardly worth the additional expense...
Others feel it is worth every cent, and/or, worthy of selling the first born or extra kidney to get.
I think the 8700K is a logical alternative currently, offering some 99% of the performance at ~80% of the cost, etc...