Several reasons, one being I wanted to make sure I would get the added value on any claim I may make
My personal experience (of adding mods on to a policy and getting quotes with mods on) is that they have accepted them and recorded them (so that the insurance is not invalidated by them) but they also pointed out to me that the accessories/modifications themselves will not be insured.
So be aware that insurers work in different ways.
If you have a BMW with all mods cons including kitchen sink, deer stalker and PTO then you might want to consult Beemerman about where he gets his insurance