The Windows clipboard manager doesn't support search. Ditto does. As soon as you open it you can just type a subset of some thing you recall copying the other day and it will immediately filter to that entry (or those entries). Biggest reason.

Ditto also rearranges the items so the last pasted is at the top rather than the last copied. This is also helpful because if I'm pasting something regularly, it's nice for it to be at or near the top.

You can configure how much history you want Ditto to store, by days or number of entries.

The Ditto clipboard entries are not as visually bulky as the Windows clipboard history entries.

The Ditto window can be resized, the Windows clipboard manager cannot be.

Those are the main reasons for me. Ditto also has a bunch of more advanced features but I've never used them.