no, you can wire a disc piezo direct to a jack and plug that into your guitar amp.
A guitar amp does have a pre amp and a power amp inside.
I guess you are meaning the preamp boxes on acoustic guitars?
you dont need one but it will tame the beast and give a bit of EQ , filter out some of the quacky harshness.
if you dont want to put one in a uke you can always make a preamp stomp box out of it . makes a versatile pedal. I think Shane Speal did a tutorial on putting one in a box.