cat is a concatenator. It's designed for taking multiple separate inputs and printing them all to stdout as a single unbroken stream. It's basically just a pipe from input to output. While it can be used to display a single file for reading, that's just a convenient side-effect. The system requirements are very low, and it exits immediately.
less is a pager, a text display application. It's designed to take a single input and break it up into pages that will fit the size of your display. You can then skip back and forth among them. This requires the program to stay active, holding the input text in memory, until you manually exit it.
And in general, two processes will consume more system resources than one.
The only real reason you'd want to use both cat and less together is if you wanted to combine several files into one and then read them as a single unit.