It's there because of the kind of partner agreement ISVs have with the cloud provider. There are two things at play:

1) How does the cloud provider know what the ISV is selling, in order to credit them for it (discounts, incentives, program membership, etc)?

2) How do the sales reps get paid on related deals? This goes both ways: the ISV's reps get paid regardless but they need a way to tag the cloud-based deal in their own CRM, and the cloud sales reps need to be able to get paid base on the ISV partner's cloud deals, and that's really hard unless there is a concrete way to relate the ISV's sales deal to cloud consumption associated with the end customer.

One of the ways to do this is on the front end via deployment tracking (like this Terraform ID). Another way is to do it on the back end through billing account linkages. In any case, the end customer shouldn't really care one way or another -- these are just mechanisms to ensure the partner relationship between ISV and hyperscaler is well understood.