Another reason for buying a tuner when you have good to great pitch is illustrated by Gordon Bok's name for the gizmo: electronic arbitrator.

Sometimes even really good musicians have slightly different ideas about the pitch (as in true vs tempered), and it has been my experience that some people like to sing/play/tune thirds sharper than others.

Dragging out the "arbitrator" at that point can drastically reduce the amount of time you spend tuning, arguing with the other musicians, and save hurt feelings.

Blessings,
Barbara