Paiggio's Feul Tank inherits its design from few of the bigger CC street bikes and has a sports style Fuel Cap, its Rubber seal works fine for some time but as soon as it gets hard it leeds to unwanted water presence in tank, to rectify this problem they inherited the same workaround which you will find in Japanese sports/street bikes as well.. aka the drain hose which drains the water while washing or raining.
Why it does not work:
Two major causes as I have notices, first is that by the time the metal hose gets rusted and becomes slow in draining which gives enough time to water to slowly seap from rubber seals into tank... The should have used a bigger wider hose which do not gets block this easily.. - POOR engineering
Secondly - Most of the Ravi paiggios are "front lowered" to make them give an aggressive look, but this makes the drain hose lose its position and water gets accumulated around rubber seal instead of going toward the hose..