Why does someone need those thick glasses after cataract surgery and why?

Cataract Surgery: Cataract surgery has advanced tremendously in the last few decades. Years ago artificial intraocular implants (iol) had not been invented ; patients were left aphakic (no iol placed). Today, some patients are left aphakic for various reasons. Thick glasses are needed (often temporarily) to give the eye the power/magnification needed. Thick glasses do not have to be a permanent condition in most.

Aphakia: Typically it is due to a condition called aphakia, where the natural focusing lens removed during the surgery is not replaced with a lens implant. This is not common. Modern uncomplicated cataract surgery does not require thick glasses afterwards, and in many cases actually reduces or eliminates the need for glasses.