If you're an HTC One A9 owner as well as a fan of CyanogenMod builds, you'll be glad to know that Cyanogen has released the Android 6.0-based CM 13 nightly build for the device, although only for the GSM model.
Agreed, the One A9 already comes with Android 6.0 out-of-the-box, but being an HTC phone, it runs the company's Sense UI on top. Flashing CM-13 build on the device will allow you to get close to stock Marshmallow experience.
In addition to the One A9, the CM-13 nightly build is now also available for the first generation Motorola Moto E smartphone. You can download the builds by heading to the Source links below, but before you do that keep in mind that nightlies aren't usually stable and installation of a custom ROM also voids your handset's warranty.
You don't, some might. That's the beauty of Android, you can install whatever you want as long as there's developement going on selected device.
I just installed CM13 on my Mi 4c - what a difference! It flies, and to be honest, it's less buggy than the weekly builds of MiUI. Only the camera is a bit crap.
I dont see any use in using the cyanogenmod for a9 as its already near stock will customisations already available. plus the main thing would be stability which you would not achieve in cyanogenmod until android n releases