If I partition the entire drive, eventually all blocks will be used, depending on how the filesystem allocates, right? So to guarantee some free space it's better to over-provision by under-partitioning. Now how do I make sure that on a used drive?
It it is also worth noting that many SSD's are over-provisioned by the manufacture anyway, in those drives manual over-provisioning might achieve very little anyway.
It runs periodically and lets the SSD know about unused areas.
So as long as you don't fill up the drive and let trim do its thing the unused areas effectively do the same thing as over provisioning.