Many corporate and government intranets were built specifically for Internet Explorer using ActiveX controls or proprietary JScript. These applications often break in modern browsers, making an old IE version the only way to access them.
The ULTIMATE Internet Explorer Collection! - Overview & Demo
Because it’s "portable," the browser doesn't overwrite the current system's registry or core files, which is vital since IE is deeply integrated into Windows. The Evolution of Internet Explorer: A Version History
Certain old industrial or scientific hardware (like telescope drivers or accounting systems) use web-based interfaces that were never updated past the IE era.