The basic benefit from having RES is that they tend to eat a lot of snails. RES are also known as "shellcrackers". Snails are a part of the lifecycle of a parasite called Black Grub or Black Spot Disease. Here's a short read on the subject...

https://www.in.gov/dnr/fishwild/3395.htm

And, a thread regarding my first encounter with the buggars...

http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Main=37965&Number=494448#Post494448

I stocked about 100 RES earlier this year in my 1/4 acre pond and have not see one since. I am sure they are their, but are either very timid, very smart, or full of snails because I can't get them on a hook. I believe they tend to bite on worms best and I have not tried them, only spinners and fake pellets.

Fish on!,
Noel