Such as enterprise level accountancy solutions, human resource packages, payroll solutions (which is the where I worked with Oracle) and so on and so forth.

A lot of the time when businesses by Oracle, they’re buying into the middleware rather than the RDBMS.

All the apps they bought.

Siebel, PeopleSoft, BEA Weblogic.

I was the DBA for a Siebel instance for ten years.

I would say J.D. Edwards, but that's AS400/DB2.

SAP is also a big driver, although they don't want to be (so much so that they bought Sybase).

J.D.Edwards old World product was AS400, but EnterpriseOne (formerly OneWorld) is also Linux, Windows, AIX, Solaris, HPUX, and IBMi and is still very much alive. Runs on Oracle DB, MS SQL Server, and DB2.

In addition to Oracle owned middleware, it's common to end up running Oracle because your vendor supports only it.

Sometimes with somewhat sensible reasons, like Oracle combining clustering and in-memory encryption, but it still means you don't get to choose...