A top-off, blowing new insulation over what's already there, is the right call for most attics we see. It's fast and it's the cheapest way to add R-value. But there are situations where the old material has to come out first, and putting new insulation over a problem just buries it.
Water damage is the first. If a roof leak or a failed bath fan duct has soaked the insulation, it's compressed, it may be growing mold, and it's no longer doing its job. Wet batts and matted cellulose need to be removed and the source fixed before anything new goes in.
Pests are the second. Rodents and other animals nest in attic insulation and leave behind droppings and urine that no amount of new material will cover. A full removal and sanitizing is the only real fix. Third is old vermiculite insulation, common in mid-century homes, which may contain asbestos and should be assessed by a specialist before anyone disturbs it.
The last two are less dramatic: insulation that's been compressed into a thin mat by years of storage and foot traffic, and batts that were installed badly to begin with, leaving open gaps between every joist. In both cases removing and starting fresh gives a better result than piling more on top.
We'll tell you which situation you're in during the on-site visit. If it's a straightforward top-off, we'll say so.