Every OS needs to have enough free storage space to use for system and software updates some of which can be very large. The OS and apps also need various amounts of storage space to allow for temporary and cache files which are necessary for the apps. Web browsers are notorious for needing a lot of storage space to deal with all the content in the multiple web pages people view. Rebooting your computer once a week can help to free up some storage since rebooting can clear out some temp & cache files a bit sooner.
I've seen macOS systems become corrupted when the drive ran out of free space due to a bug in macOS which started over writing files and folder information.
If you are using HS or Mojave with the APFS file system, then you can lock yourself out of the OS because the drive because it may no longer be possible to mount the drive due to the way the new APFS file system works. There are several posts on these forums where this has happened to a few users who ran out of storage space. If you use Time Machine or other backup software, the backup software will take temporary snapshots of the systems which can use up considerable space if you are making a lot of modifications to the files.
SSDs also perform better and last longer if the SSD has extra free space on the drive.
Unfortunately a 128GB SSD actually has only about 120GB of usable space. Most technical users will say that 120GB SSD is way to small for almost everyone today, but it is fine for users who are willing to use external storage to store their large files (pictures, music, videos, etc.). It all depends on how the system is used whether 128GB SSD is sufficient.
Don't forget to empty the Trash to free up disk space and clean up the Downloads folder. Does your iOS device(s) sync to the computer, taking up valuable storage space in the ~/Library?
