Well part of why in my opinion is that it would help alleviate some of exactly what you wrote you are stuggling with.
If you know the person and have a friendship with them yourself it's easier to not imagine that they are replacing you or trying to usurp your position in your significant others life.
I understand the theory that if you don't want to date them you don't need to know them, but the reality of that is-you don't KNOW what the situation is which lends itself to jealousy of non-existant issues and insecurites and fears and....
You get the picture.
"Love As Thou Wilt"