Hi cressidom,
the water draining of the fuels tanks have two main reasons:
1 is to avoid any bio contamination as NutLoose has explained before. Several kind of fongus can growth up in the tanks and after a while, very big troubles can occur during flight ( i.e; fuel filter blocking with power loss or IFSD). to prevent that, a lot of A/C makers highly recommend to add some biocidal solutions like Biobor recurently, depending on where the A/c operates. the worst is in tropical condition, with high condensation in the tanks and high moisture rate witch help the fongus growth up.
the second is to avoid ice formation in fuel tubes at high altitude with all associated troubles.