Hi,
IMAgent starts up when the computer does.
It provides the "Listening" function of Messages for off Line IMs in AIM and Jabber accounts
It also responds to iMessages that are sent.
Therefore it is always connected to the 'net unlike mail that only connects when it fetches or a browser that only does so when you change pages.
If the iMessages account is not used and the setting in Preferences > General Section is set to "When I Quit, set my Status to Off Line" then the usage will drop.
NOTE
Status only applies to Buddy List accounts (AIM, Jabber including Google and Facebook Chat IDs and Yahoo plus Bonjour)
The iMessages account has not status.
Ergo the setting does not stop iMessages showing up on the DOCK icon if the app is Quit.
9:59 p.m. Wednesday; October 7, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad