Manual Motor Starters - Education Required!

[Taylops]

Can anyone explain the use in "manual motor starters" in switching pool filter and cleaning pumps on and off? I have a fair knowledge of electrics and electronics in general (being an electronics/telecoms engineer) but have not come across these devices before. The system on my pool consists of two such devices each associated with a timer and ELCB to control the main pump and cleaner pump. They both have various controls on the front which are "alien" to me........ the type number of both is GPS1BSAJ (GE)

Thanks for any education that can be provided!