There are many reasons.

* Preprovisioning - devices have the right certificates and know about your corporate networks. They have the necessary apps and just work.

* Tracking - if a device is lost or stolen, monitor where it is and remotely lock or wipe it

* Monitoring - have a log to audit if someone does something malicious

* Security - reduce the chance of your employees installing malware, spyware, etc. whether by accident or intention

* Locking things down - put gates in the way of bad actions like copying sensitive data into public apps or clouds. Even if you're unable to block everything, attempts to block remind honest employees and provide strong evidence that anyone who proceeds was intentionally violating policy and should be fired.

Etc., etc.