What is the economic interest of a customer who buys software or cloud services, with regard to IAM?

Question

Accepted Answer

To keep both the number of IAM systems in use and the number of required technologies as small as possible.
Every product that comes with its own built-in IAM adds cost for the buyer: another user list to maintain, another set of roles to define, another place to remove access when someone leaves, another login for users, another technology the IT staff must know.
The customer therefore wants:

Few IAM systems: ideally one central IAM to which every product connects, instead of one island per product.
Few technologies: ideally a small set of standards (for example OIDC or SAML for login and SCIM for provisioning) that all products support, so that skills, tools and processes can be reused.

This interest gives customers leverage over vendors: by making "supports our standard login and provisioning interfaces" a purchasing requirement, they push vendors toward standards and keep their own landscape manageable. It also explains why a vendor with a proprietary, hard-to-integrate IAM risks losing deals.
Go deeper:

Shadow IT (Wikipedia) — what happens when cloud services are bought outside central IT and its IAM.