Krill is a good source of Omega 3 of course. Most commercial foods (so I have read) are not balanced as far as omegas, containing too much omega 6. So adding a krill or salmon oil supplement is beneficial that way. It helps keep skin and coat healthy and is a good source of vitamin D (mainly of concern for raw feeders)
Krill, specifically is a very good anti inflammatory.
There's probably more but it's been a while since I did that research. I'll have to look for some links.
Most cats don't take to it readily. Jennie did (she eats anything) but Mazy and Queen Eva needed some incentive dusting for a while. Now they eat it fine.