DEPRECIATION ISSUE

why wouldnt a company wants to integrate depreciation book with GL

DeprBook MUST be integrated in GL, otherwise no amounts of any transaction involving FA (purchase, depreciation, disposal and other) will be reflected in corresponding GL accounts.

The option to include or not is there only for one reason - NAV allows to maintain more than one DeprBook simultaneously.
You can create as many as you wish, but in real life I’ve seen max 2 - one set up as accounting (in fact, country’s tax regulations) rules require, the other for corporate inner use.
If you have more than one, you choose which one is integrated, but one of them MUST be chosen.