Here is a link to a very good laser boresighter.
I have several rifles that all have picatinny rails on them. I have several scopes all in QD bases and I move them from rifle to rifle as the mood strikes me.
What I have found is that the aiming point can change a lot when switching a scope from one rifle to the next. It is true I could boresight looking through the barrel but I choose not to. I change out scopes and put the boresighter in and dial in the 25 yard zero very quickly. Do you have to have one? Probably not but they do make things easier for me.