Naturally dive boots provide protection for your feet both topside as well as during the dive. They protect your feet from not so friendly shore entries as well as provide a layer of protection from possible stings and bites due to marine life you may unexpectedly encounter on your dive.
With all of that being said, dive boots provide a level of comfort for your feet. They protect your feet from chafing due to direct skin to rubber contact with your fins as well as provide warmth for your feet when on colder dives.
Dive socks offer an alternative to dive boots if you find the bulkiness associated with boots to be undesirable.
Diana Doreian explains the advantages and disadvantages of different types of dive boots in the video below.