you need to link/join ‘situations’ and ‘people’. ‘Which’ is a relative pronoun and refers back to the noun before it. If you said: situations which, you would then have to describe those situations but the next word is ‘people’ and is the subject of the next part of the sentence. Obviously that doesn’t work and so we need a word to describe the position that these people are in. That’s the reason why we say: situations where … Another possibility is 'situations in which …