Hi Donny,
You don’t necessarily have to play them.
They’re part of the style. The majority T4/T5 styles that didn’t work correctly on psrs, now play almost perfectly on the sx900.

Ok, so maybe not into prior styles, but there’s over 100 T5 styles missing from the Genos, so there probably would be that number missing from the sx900. Some good ones too. Yamaha must have thought it important enough they did the missing T5 styles as a download for Genos.

A number of the onboard R&B styles have voices, brings the style to life. Other categories use some too.

They don’t need to be played as a melody voice, they’re part of the style.
If you listen to a sx900 style that uses them vs the same psr style. No comparison.