If you are on High Sierra then it's TimeMachine fault. That's the solution that worked for me.
MacOS High Sierra: Everything you need to know about Apple’s latest Mac operating system. Mail has been optimized so it uses less space on your storage device. IOS Files: There’s a good chance you can quickly clear up a lot of storage space by deleting old iOS backups and installers. If you don’t need to keep old backups around, go ahead and delete them.
Type this command in you terminal: sudo tmutil listlocalsnapshots / to check the snapshots of TimeMachine. You get some strings like these: com.apple.TimeMachine.2017-39 com.apple.TimeMachine.2017-07 com.apple.TimeMachine.2017-17 com.apple.TimeMachine.2017-53 com.apple.TimeMachine.2017-13 com.apple.TimeMachine.2017-34 com.apple.TimeMachine.2017-54 You need this command to delete TimeMachine mess: tmutil deletelocalsnapshots 2017-34 Type this command for each of those snapshots (changing the date) and you'll get a great amount of free space! I believe that when I got my MBP, the size was around 16 GB. One night the size of the 'System' section of my storage increased 40 GB in an hour (though, I am running the High Sierra Beta).
I restarted a few times and nothing seemed to happen. I read an answer somewhere to an issue related to this that I could delete the com.apple.coresymbolicationd folder in /System/Library/Caches/. The system couldn't read the size of the directory so I figured it was a problem folder, so I deleted it, emptied the garbage, and restarted. Now my System section is 50 GB (used to be 120 GB). Gave me different sizes than the System Information window and it didn't find any problematically large files., however, did find a glob of ~75 GB that it couldn't scan because of permissions, even when scanning as admin.
The com.apple.coresymbolicationd directory has restrictive permissions, so I assume that was the culprit. Deleting that directory isn't harmful to the system as far as I know, and it regenerates when the OS needs it again. You can also look at the built in Disk Utility. That gave me conflicting reports each restart but did inform me on one restart that ~100 GB of data was purgeable. I had the same problem.
My System folder was 50 GB, my Applications folder was 20 GB, and my Downloads, Music, etc accounted for about 20 GB. And my 'About this Mac' was showing 'System' space used as 160 GB, thus showing that there was no disk space any more on my 256 GB mac. So I spent some time found where this '160 GB' was being used. In the Users//Library folder, one of my application had created a directory and was storing some files over there which accounted for majority of the 160 GB. I removed that application and deleted this folder which resolved my problem. 
Note that deleting anything in a system folder can be harmful, so only do it if you know w.
Store Files and Photos in iCloud RELATED: iCloud can now automatically upload and store files in your Desktop and Documents folders, as well as photos. Enable this option and your local copies can be automatically deleted from local storage to free up space. Only recently opened files and optimized photos–smaller versions of the original larger photos that remain stored online–will be stored on this Mac. To configure this, click the “Store in iCloud” button. You can also find these options at System Preferences > iCloud. Use the Options buttons next to iCloud Drive and Photos.
You’ll need space available in iCloud to do this, so you may either need to or purchase more storage for a monthly fee. If you plan on storing a lot of files in iCloud, you’ll definitely need to pay a monthly fee. ICloud only gives you 5GB of storage space for free, and that’s shared with your iOS backups. Optimize iTunes Video Storage iTunes can “optimize” local storage of movies and TV shows.
For example, even after instructing Chrome to let the site open pop-up windows, most of those windows are far too small and have to be resized manually. But on the whole, the online version has been fine. Is quickbooks for mac compatible with quickbooks for pc. Check printing works, but you can adjust only the overall alignment, not the location of individual form elements—some of which don’t line up correctly on our check stock.