When it comes to self-publishing wide with Amazon KDP and an aggregator (in this question it was Draft2Digital, but the same applies for IngramSpark, LuLu, etc.) you’ll want to own your ISBN.
The “Free” ISBN from Amazon KDP can’t be taken to the aggregators
Same goes the other way around
For ultimate control over which platforms you post your books to and the ability to remove them or move to another platform you’ll want to own your ISBN.
(As an aside, the ASIN is something every product on Amazon has. Towels, books, shoes, etc. Your ISBN is not linked to the ASIN.)