Why solvent cleaning of clean wafers includes de-ionized (DI) water?
Acetone is hygroscopic.  So is isopropanol.   Neither will be completely
devoid of water, so water must always be removed by evaporation.
The difficulty is, if any ionic solids are present, THOSE
would remain after evaporation (and if those solids are hygroscopic,
the 'drying' would be temporary, a little conductive saltwater puddle
can reoccur later).  So, the rinse water needn't be 'pure', a little alcohol
in it won't hurt.  But it oughtn't be (for instance) salty.

Deionized water is cheap and disposable, and (in conjunction with an
'air knife' or other droplet-removal scheme) mobilizes ionic contaminants
and removes them from the wafer.