Answer:
Rhodium and platinum as precious metals (not being corroded)
are used for jewelry;
for automobile cats - the Pt helps with complete fuel burning,
the Rh prevents Nox's. If electric car develops, this use will be much lower.
Pt is a catalizator in hydrogen reaction with oxygen, e.g. in fuel cells
Pt is helping water electrolysis by lowering the over voltage of hydrogen
evolution, so better output
For same reason, Pt (black) is used in hydrogen electrodes
Pt, hard and good conductor, is used in old automobiles ignition, sparking systems.