Isolation Transformers

Why buy a transformer that changes nothing

Because the windings are not electrically connected. Power crosses the gap magnetically, so the secondary has no galvanic path back to the source. That breaks ground loops, blocks common mode noise, and lets you establish a separately derived system with its own grounding point. A 480 to 480 volt isolation transformer does all of that while leaving the voltage alone.

Where it earns its money

Sensitive electronics and instrumentation, medical equipment, test benches, and anywhere a new neutral to ground bond is needed away from the service entrance. It is also used to convert a delta system into a wye system so a neutral becomes available, which is a change of configuration rather than voltage.

Every pad mounted transformer is already isolating

Worth saying plainly. A standard two winding distribution transformer provides the same galvanic separation. Buying a separate isolation transformer downstream of one is only worth it if you need a second separately derived system or a shield between specific equipment and everything else.

What is a 480 to 480 volt isolation transformer for

The voltage is unchanged, so what you are buying is galvanic separation between primary and secondary. It breaks ground loops, blocks common mode noise and creates a separately derived system with its own grounding point.

Is a pad mounted transformer an isolation transformer

A standard two winding pad mounted transformer does provide galvanic isolation. It is not marketed as an isolation transformer because its purpose is changing voltage, but the separation between windings is the same principle.

Do I need an isolation transformer if I already have a distribution transformer

Usually not. The distribution transformer already isolates. You would add one only if you need a second separately derived system, a different grounding arrangement, or a shield for particularly sensitive equipment.