If you are running out of diskspace constantly on your mac, it might be interesting to know that on high sierra, it automates backing-up timemachine files on your local mac; that is if your external drive is not connected. Mac will copy and delete those files, the next time when you connect your timemachine designated drive. I found myself deleting files off my drive only to find my hdd available space miraculously disappearing again. On running disk utility, its clear that mac takes backups called snapshots.. and it takes a whopping 6 of them costing me around 110gb.
The solution is easy, either plug in your timemachine and let it run by going to timemachine -> backup now
or alternatively open your terminal and run

tmutil listlocalsnapshots /

you can then proceed to remove the snapshots with the following command, replacing the date stamp with the ones you see above.

sudo tmutil deletelocalsnapshots 2018-05-01-XXXXXXX

Nevertheless, the first option is still the best if you are not comfortable fidding with terminal.