Question about Tachs and Encoders on DC motors

I understand how DC Motors with encoders can be servo controlled, but
I am not sure how DC motors with tachs are used.

I also noticed at work that our Bridgeport EZ-Track has SEM DC servo
motors that have both a Tach and an Encoder on them. Why are both
needed? And how is this used in a control scheme?

Any hints or info would be appreciated.


Sincerely,

Paul Rooney