From what I see some Sambo fighters practice BJJ too, Khabib Nurmagomedov for example. Why would they do that?
Presumably, they do so for the same reason most MMA fighters cross-train, to increase their breadth of techniques and to get practice fighting against other styles. For example, an often under-appreciated benefit to cross-training with other styles is that it gets you to try techniques against people who don't know how the technique is "supposed" to work, which can help eliminate magical thinking about the efficacy of a given technique. The classic example is people practicing "no touch" martial arts whose powers only really work against their own students, but even in more practical styles, students who had an initially painful response to a particular technique are more likely to tap out early or not look for counters, because they really don't want their elbow dislocated again.
There is a lot of crossover between the different grappling arts.
& when people fight, they more often than naught have a specific "game" they play. Some prefer leglocks, and some go more for chokes. Some play defense, others... you get the idea.
By training in different styles of grappling, practitioners can expose themselves to different techniques which they can then add to their own specific style.
Ever person fights a little differently depending on their body-types & experience level; as well as, who they're fighting. Cross training is a great way to pick up new tricks that are outside of (and even complimentary to) what you'd normally practice.