There's a fine balance between protection/durability and resonance when applying a finish. If you put too much on, it will make the guitar's wood not ring as much. But if you don't put enough on, you can end up with places that wear out over time. Most cheap guitar makers automate everything, and you end up with either a guitar that's got bare wood showing after a year of playing it, or a guitar that sounds like a ukelele because it's been gunked up with too much finish! That's one of the reasons I do all my finishes by hand.