If you put a load of time, effort, and/or money into inventing a Better Mousetrap, you'd want a return on that, wouldn't you?

You wouldn't want some fly-by-night to just come along, copy your invention, and then make loads of money from it, with nothing for you - would you?!

That was the original idea behind the Patent system: it allowed you to publish your invention - to the benefit of all - while still giving you the legal right to profit from it.

Yes, these days big companies can use it as a way to just keep the competition out of their market, and lawyers make big money by obfuscating it so as to be unusable to anyone else while also being so general that it'll catch anyting even vaguely similar.
But the basic principle is a good one.

To qualify, the idea has to be genuinely Novel - ie, a true invention - not just an application that would be obvious to anyone "skilled in the art".

Different jusrisdictions differ on what they will allow patents on; eg, in Europe, you can't patent software.