When fish started developing the stronger limbs needed to walk(Slither?) onto land how was this beneficial? I cannot think of any form of food that would be helpful that would not be plentiful in water, but it must have been so as from what I can tell most beneficial mutations tend to be either a better way to either survive or obtain nutrients.
Insects were the first animals to conquer land. Vertebrates followed later. So there may not have been an absence of a food supply. Also, the ability to leap to land for safety to avoid a predator is useful if said predator cannot follow. similar in manner to flying fish.
Walking developed before fish moved onto land. Fish that live in shallow water crawl along the bottom instead of swimming, because it is easier to sneak up on prey and hide from predators that way and it disturbs the water less than swimming. There are several modern fish that do this.
Like many brackish water fish they also could already breath air. Brackish water tends to be low on oxygen while air is always high.
Resources may have been plentiful in water but so was competition and predators.
For vertebrates there were no predators on land, there was food that no other fish were eating, there was access to new pools of water, etc. It was a completely open environment full of resources that had not evolved alongside them (thus had few defenses), no competition (the advantage of being the first vertebrate on land) and it was completely free of predators that is the like eden for an organism. There would be a huge selective advantage in exploiting it. You may want to familiarize yourself with adaptive radiation in general if you don't understand this.