Why does a bus need kickdown and hold gears?

m'dina bus (bus 1151)

chouha au terminus 65

chouha au terminus 65
Nice Slipping ZF and kick-down!! Iveco Eurorider27A (bus 1192) part 2

Nice Slipping ZF and kick-down!! Iveco Eurorider27A (bus 1192) part 2
Nice ZF 6HP Ecomat !! Zhongtong Sunny at Rabat capital of Morocco

Nice ZF 6HP Ecomat !! Zhongtong Sunny at Rabat capital of Morocco
Full Kickdown ZF !! Renault R312 (Bus 6164) EX-RATP 6486

Full Kickdown ZF !! Renault R312 (Bus 6164) EX-RATP 6486
Why does a bus need kickdown and hold gears?

Why does a bus need kickdown and hold gears?
Berliet / Renault PR + Renault R312 (1976-1988)

Berliet / Renault PR + Renault R312 (1976-1988)
Renault R312 (Ex-RATP 5366) BV VOITH à Sidi Othmane

Renault R312 (Ex-RATP 5366) BV VOITH à Sidi Othmane
Nice acceleration and kickdown !! (bus 1034) à Hay hassani,Casablanca

Nice acceleration and kickdown !! (bus 1034) à Hay hassani,Casablanca