They do, but that's not even the primary reason for x86 Android apps. Android developers on e.g. Windows will test their apps during development, so the app will normally get compiled for x86 to use on an emulator on their PC. The ARM version is only required for the final published release, or testing on a real ARM device. But since x86 is the "default" in the development process, there is little reason not to send both to Google Play, since you are making x86 builds anyway for your own use during development.

Nowadays this process is mostly transparent for developers, they just write an app and Google Play handles splitting it by architecture, so most apps have an x86 version even if the developer didn't do anything.