For the same reason that your flashlight needs two batteries.
A set of zinc and copper metals will move electrons with only so much oomph. That’s because there’s only a certain amount of difference between the electron-holding powers of zinc and copper.
But if you need more electron-moving force, to light a bulb, for example, you can connect a second set of zinc and copper metals after the first, giving twice as much kick to the electrons.
The technical word for electron kick is voltage: the force with which the electrons are made to move. The zinc-copper combination makes about 1 volt of kick.
If a particular clock needs 2 volts to run, you’ll need two potato batteries connected together.