The theme from LG for the LG G3 was "Smart". There's the Smart Keyboard, Smart Notice and a bunch of other things. Basically they wanted to keep everything minimal but still smart. And I think they've done a great job in that regard in a lot of areas. One is Smart Cleaning. Which you'll find under the General tab in Settings. Smart Cleaning is actually something I think all OEMs need to add into their device. So we don't have to download third party apps like Clean Master, Ccleaner, etc. Basically what Smart Cleaning does is it finds all the cached data and temp data on your phone and allows you to easily remove it.
Now there are three sections inside Smart Cleaning. There's Temporary Files which is full of clip tray temp data and cached data. There's also the Download Folder. So if you download a lot of things, maybe some APKs, or wallpapers, etc., this is where you'd find how much space those are using up. Finally there are Idle Apps, which are apps that haven't been used in a month. LG will tell you which ones those are and allow you to disable or uninstall them, if possible. On this screen, it'll show you how much each section is taking up on your G3, also how much space you've used altogether. For example, in the picture above, I'm using about 13% of my space, or 3.48GB. And I can clear 527MB of space using Smart Cleaning.
To access Smart Cleaning you just need to head into Settings > General > Smart Cleaning. And from there you can go ahead and get some of your storage space back. It's actually quite simple to do. Of course, apps like Clean Master, can do more things and probably get more space cleared out, but this is a good start.