Composer provides an easy way to separate and manage the tasks of creating, updating, showing, hiding, and eventually destroying game interfaces / scenes.
That’s it.
If you don’t need it, don’t use it. However, folks generally don’t want to code their own scene management and tweening code, so you’ll see a lot of folks using it.
Personally, I believe users should not use it till they know how to make their own game modules and have experimented making their own scene management code.
I all too often seen new Corona users jump into using composer.* like it is a requirement. That, added to their ignorance of: Lua, Corona in general, scope and visiblity, … adds up to mistake after mistake and some terribly written game code.
Better IMHO if they had stayed with just config.lua, build.settings, and main.lua, then worked up from there.