Windows Mobile Device Center/ActiveSync are legacy applications made by Microsoft. They provide an access to file system, registry, … let’s say, they provide full remote access to connected phone’s features.
What's this launcher for?
Main purpose of this application is to enable Remote API support on device. PC will detect phone as Windows Mobile and Windows Phone 7 device simultaneously. Zune will still work.
File managers. Almost every file manager that has WM support will work on WP7. Nice examples: Windows Explorer, Resco SendTo plugin (part of Resco File Explorer installation), CE Remote Tools File Manager, Altap Salamander, etc
Registry Editors: CE Remote Tools Registry Editor, CeRegEdit, Registry Workshop.
Most of CE Remote Tools work.
Many more apps are supported.
There is no contacts/file/calendar/etc synchronization. PC may still offer it but it won't work.
Note about debugging
IDA debugger also should work after a bit of editing magic.
Custom ROM usage
In 2012, the usage was free but limited. You had to ask me for permission (it was for tracking, it was given all the time).
As of now, usage in custom ROMs is free and unlimited.