"I can't figure out why you would remove then replace insulation."

So they can loacate ad seal the leaks.

The insulation can provide some god clues for larger leaks.

The insulation will trap dust and dirt from the moving air and there will be dirty spots in the insulation where the air moved through.

It has to be pulled back (at least) to then try to locate the actual leak and seal it.
If there are a lot of significant leaks it might be easier to just remove the old insulation, seal the leaks, then put new insulation in place.

Thanks for the reply. I just assumed it could be moved around in order to seal. I can see it being easiest to remove.

"I can see it being easiest to remove."

Moving it around will tend to remove any loft it had, reducing its R value.

It often is a matter of scale.

With a lot of leaks I typically would take pictures to capture the leak spots showing in the insulation, then remove it and use the pictures to help locate the spots again to ensure they get sealed.