Yes. Or in case you get so excited by the awesome performance you wet yourself. Or in case the aeroplane is caught in a rain storm and a significant amount of water makes its way into the fuselage. Or in case, while washing your plane, water makes its way into the fuselage. These things can and do happen. Well... maybe not the first one.
People have been caught out by weight and balance changes after water has collected inside then rushed aft on take-off. Never seen it happen on an RV as we tend to hangar them but certainly seen tide marks in spam cans. Allowing water to escape is a good thing.