HUD can do most things more easily, but many functions can be used via your phone. However, there are a few features that can only be used if you are running HUD:
Record calls on-the-fly. Requires HUD Agent or above.
(you can still schedule automatic inbound queue call recording without HUD).
Visually see who's on calls. Requires HUD Team / Enterprise or above.
(excepting the Busy Lamp Field on certain Aastra and Polycom phones, also known as BLF)Barge/monitor calls.Update: Dialing *66 (*MO) plus a valid extension number will allow you to monitor the call that extension is making or the next call that extension makes. The users on the call will not be able to hear you. It is permissions-controlled; you must have the HUD - Barge permission over the extension you're monitoring.
Reference
HUD Team / Enterprise edition (and above) video introduction / tutorial.
HUD Agent / Call Center Edition & Queues - video detailing features specific to these versions. We recommend watching the Team / Enterprise video above first.