why is it important to know how to separate solids from liquids

Answer 1

I think because a "solid" could contain many molecules and once it's separated via some techniques, that solid becomes two different molecules as a liquid.

Answer 2

I think the response by Collee2 is hogwash. It makes no sense what-so-ever.
There are several reasons to separate solids from liquids. One is that you may wish to have a "pure" solid or a "pure" liquid.