Some people have sub jobs. Like me for example, I main MCH/BRD and my sub roles are SCH/AST. Which can be handy.

Example:
Our tank could not make it, so one of our healers will tank in his place, and I will take the healers place. We can get a random DD from PF or a friend we trust.
I believe most statics prefer players than can play multiple roles for such cases.

But I don't want a specific role to change from DD to a healer or Tank based on the gear they equip. Just level another role and learn to play it.

The closest thing we have to this is ACN which can become SCH (Healer) or SMN (DD). The developers admitted that this was a mistake they made and we won't be seeing something like it again. They share the same base class abilities but different role abilities and gear.