Let’s say there are 4 candidates, A B C and D, and a large group of people have them in that order of preference, their (honest) acceptance would be A and B, but they’d much prefer C over D if those were the only two options.
A prominent forecast comes out and predicts a tossup between C or D. They all act in self-interest and strategically list A B and C as approved, to lower the chance of D winning over C.
Now that forecast was wrong about A’s low chances for whatever reason and had they solely and honestly put down A and B, A would’ve barely won. All of them adding C doomed them to have to put up with someone they don’t honestly approve of.
As you said before though, if we take this scenario into a single vote fptp system, we have all of them giving their single vote to C. Not only does this harm the chances of A winning even more, it also reinforces never voting for A as “A doesn’t have a chance anyway and voting for A would be a wasted vote”.
You can also construct a similar scenario the other way around for leaving out a candidate the group would approve of.