When to use a slotted vs. solid turner?
It's largely preference, and sets may include both to seem like better value, but there are sometimes reasons to choose one or the other.

Slotted: allows more fat (or water) to fall off the food; more flexible for the same thickness/material so can slip under things better.

Solid: supports crumbly or loose foods better (e.g. lifting out finely chopped veg). Extra stiffness allows it to be used to break things up in the pan and to support heavier foods, without being overly thick.