It’s infrastructure for SaaS apps to easily connect to all the different systems used in the enterprise, including Okta, OneLogin, ActiveDirectory, Workday, etc.
It's rare that a company will use multiple identity systems, but there exists lots of fragmentation across the large companies, which makes building a universal solution into your app very time consuming.
Active Directory: holds those accounts.
Okta/OneLogin: facilitates use of AD accounts to log in to webapps.