For a start there are several services that depend upon the Plug and Play Service:
Windows Biometric Service
Windows Driver Foundation - User-mode Driver Framework
Internet Connection Sharing (ICS)
Routing and Remote Access
Remote Access Auto Connection Manager
Remote Access Connection Manager
Tablet PC Input Service
Windows Audio Endpoint Builder
Perhaps one of these services needed to start?
The Plug and Play service is an integral part of windows 7 / 08, if you are really interested to know why it exists, what it does and what may cause it to stop and start, then you could do worse than to obtain and read Windows Internals by Mark Russinovich.
TL;DR - Since you don't know what this service does, or how it relates to the OS as a whole, it's probably best if you don't speculate about whether or not it should be running.