Shelves under the bench allow for storage of heavy items, also adding stability to the bench.
"Boxing" it in (I think thats the proper term) with plywood, even just 3/8" , on the sides and back also helps to add stability.
The National Reloading Manufacturers Assn or some such organization...had a nice set of plans for a heavy duty bench out for like five bucks.
I built one of those back in the 1970's and it's still as solid as a rock. Boxed in sides, 4x4 legs, HD shelf underneath, bolted together, etc.
NICE use of the space BTW.