Lets all go back to eighth grade science class here.
Condensation!. Condensation accumulates as air temperature fluctuates and is a major killer of gasoline. Depending on your climate it may not be a problem. I live in an area with fairly high levels of humidity.
Added:
Given that filling stations use in-ground tanks and ground temperature does not fluctuate like air temp, I would assume gasoline would last longer in them than in your vehicles tank or your jerry can. So, depending on your situation, water in your gas could be a big concern. I kinda have my doubts about a filter but what do I know, maybe it works. I read you can add isopropyl alcohol to gas to counteract the water.
Do some more research on that before you go pouring rubbing alcohol in your tank