With the BT system technically you can transfer calls, have calls transferred to you and make calls as if you were connected directly to the internal phone system (because you are)
Switchboard operators (remember them?) and geeks aside, I don't know anyone who has ever been capable of transferring calls with any confidence—often the last thing you hear before random button-stabbing takes place is "Sorry, if this doesn't work then call her on this mobile number..."
This is solution for those rare confluences where someone:
1. takes a call on a landline,
2. wants to pass the buck,
3. can't just look on their move and say "Sure, here's her mobile number."
4. has the PABX number to hand.
How often does that happen at yours?