In the way battery works, redox reaction is taking place. My question was why does the oxidized electrons (after being used to power motor, ect...) was used to reduce another substance when instead we can keep using the electrons to power another device? or can we?

In a DC circuit the electrons have to return to the source after generating work at the load. I you add another load then you drop the voltage across both devices so you lose functionality.

With a 24 v supply you can use one 24 volt motor or two identical 12 volt motors but if you add another motor the voltage drops at each motor and they all slow.

In the way battery works, redox reaction is taking place. My question was why does the oxidized electrons (after being used to power motor, ect...) was used to reduce another substance when instead we can keep using the electrons to power another device? or can we?

 

You have to undo the redox reaction to use the electrons that are present in that final compound. Batteries will eventually discharge; there's a finite amount of reactants present.

why will the voltage drops if you add another load? Did the velocity of the electrons slow down?