When to insert B roll?

Hi

I have been watching a lot of short movie featurettes on youtube and have a questions about this style of editing. Im trying to understand when to insert b roll and when to cut to a speaker.

I hope this question isnt too vague. Im aware it depends on the tone of the piece, as well as covering up mistakes or jump cuts when the interview is trimmed of unnessary words or pauses as well as generally keeping things smooth etc.

But creatively when is a generally good place to insert B roll when somebody is talking? Is it usually a casr of illustrating the points the speaker makes?

Also sometimes they will cut back to the speaker after b roll, before the next point is made, why is this?

I know there obviously isnt a definitive answer by any means, but just a push in the general direction as to what I should be aiming for when editing would be very helpful and appreciated.