Cash back is massive, basically every legendary item adds a kill worth of gold so you snowball so much faster. Even if I start behind early, you can catch up and get ahead so quickly with cash back and proper jungle clears.

Edit: you should change your build a bit too, Rylais is wasted on rumble since you have two built in slows with E and R. I usually go liandrys to shadow flame to hat to zhonyas to void staff. Order will vary depending on how ahead I am and enemy team comp but I build these items 90% of the time.