Vort: Glad you asked.

Here, "imagined sins" include: (1) the sins that the others think they have committed themselves, (2) the sins that we think others have committed, and (3) the sins that that third parties think the others have committed.

Why should we forgive imagined sins?

Speaking for myself...

I & others have thought, said &/or done some things at times, that I then or later disliked, classified, complained about & resolved one way or another.

My definition of what constitutes a "sin" usually includes the element of neglect or negative motive. However, I avoid evaluating other's possible neglect or motives. Thus, I avoid imagining sins, avoid judging persons.

Nevertheless, according to Biblical commandments, we are all sinners. I choose to forgive, whatever the case.

Should we worry about our imagining sins?

More globally, I would say that we should be concerned (versus "worry") & watchful about many things, to reasonably question ourselves, to resolve what is worth resolving, to ultimately treat others as we want them to treat us.

Of course, there's much more behind all above.