Replacing fork seals if often one of the first things a rider starts doing at home, and fitting everything back together (often “in the reverse order to which it was disassembled” to quote some manuals) can be rather tricky.
Fork seal drivers won’t help with bushings, springs and other fiddly bits. But they do make getting those oil seals in good and proper a great deal easier.
More importantly, by applying equal pressure across the seal surface there is much lower risk of damage to the seals (which of course, means taking everything apart all over again) than with other methods.
As these drivers are adjustable for fork sizes 26-45mm, when it’s time for the old bike to switch hands there’s no need to splash out on new tools.