If an inbound call is routed to a phone, it rings for a defined ring time, and then the call disconnects. For example, an incoming call rings to phone 101, and since there are no "Try First," "If Busy," or "If Unanswered" options, it will will drop off if not answered.
When routing to an extension, users have configuration options that allow the call to be routed to "Try First," "If Busy," or "If Unanswered." If an inbound call is routed to extension 101 it will "Try First" phone 101. If they do not answer, it will route to the "If Unanswered" option which is commonly a mailbox or a "Find Me" option. Most often, the "Find Me" routing will go to the users cell phone.
Routing to an extension gives the user more options than routing to a phone.