There are many reasons for having excess storage capacity other than running out of space for your files. A lot of it has to do with operations that perform best by having capacity overhead like snapshots, defragmenting, data scrubbing, short stroking, and others.
With SSDs it has been recommended to leave around 20-25% free space to maintain optimal performance and to ensure that the SSD has enough space for efficient garbage collection, wear leveling, and TRIM operations. These figures are ballpark rule of thumb and not set in stone. YMMV.
4x4TB WD Red HDDs. All are over 6 years old now with no issues.
