If you keep a server running it gradually degrades over time (I think) and so restarting the client refreshes it guess?
It might be on Windows which is why it restarts frequently (some server geek like @Zeito can tell you about Linux and Windows OS servers AND tell me why too!)
Typically server do not always need to restart and they are designed with long uptimes in mind.
I don't know the particulars of how the teamspeak server applications works but it could be to clear out a cache or commit lots that build up over 4 hours of heavy operation or something similar.
Not to mention things like patches etc. Which may restart the machine rather than just the app.