I have had the misfortune of having to get D&B numbers (for various Apple things). I believe is the source for lead lists where you start to get dozens to text and phone spam calls per day. Do not pay hundreds of dollars for this if you can at all avoid it.

Definitely avoid unless you are distributing a consumer application through the dominant app stores (App Store and Google Play) ~globally, in which case you may not be able to avoid (or avoiding will be just as much work).

Google and Apple require it for lots of mobile apps targeting certain consumer segments because some countries (eg: Brazil, IIRC? don't quote me on that) have chosen to use D&B as a qualified unique identifier of business legitimacy and it requires exposing personal information of your company's leadership to them.