Graphite and tubes...

What kind of advantage if any is graphite compared to whatever else are plates made of?

I am looking at this pair of RL12 T15 for my amps, and was wondrin'

RL12T15 Tekade graphite

Main advantage of graphite is higher possible temperature. Some tubes can operate at graphite plate red hot. But special tube design is required for that to be safe. Usually these are transmitter tubes. But you still need good (usually forced) cooling. For audio applications there is a little value from graphite plate.