Whats the point of physically docking there though? You can do all that wirelessly.
The same reason people often still use ethernet versus WiFi for desktops and laptops. If you are charging your phone anyway it doesn't hurt to have a potentially more stable/higher bandwidth physical connection, especially if the phone is essentially outputting to an external display. Docks were always for attaching a device for power, using an external display, networking, and adding extra inputs (USB, keyboard, mouse, etc). With Continuum you can choose to just wirelessly do the external display part (which you can do now anyway via casting, which is labled "connect" in the Windows 10 action center, but Continuum makes the alternating between phone and desktop modes more automatic and seamless) as well as use a wireless keyboard and mouse, but nothing beats having a physical connection to at least fall back to.
With a dock you could turn a Windows 10 Mobile phone into a full-fledged desktop, albeit not a powerful one. Still, there are lot of people whose computing needs are currently served by an aging desktop or laptop, or who are currently using basic use computers like Chromebooks. A higher end Windows 10 Mobile phone would probably be fine serving as that kind of computer. No one is expecting power users (like most of us) to get rid of their custom-made, purpose-built workhorse PCs.
What Denly is describing sounds exactly like what Apple calls Continuity (Handoff specifically, which is a Continuity feature). You wouldn't
need physical docking to do that, either. I expect Microsoft is going to add more features for syncing messaging and controlling phones through action center/notifications and maybe eventually a separate app.