The overflow tubes allow air to be vented out as air fills the tank.
Like a siphon, however, once water starts to draw out of it, if will drain water back out.
Lots of JOF members have added a shut off valve or cap on the end of the drain hose
Jim & Kim from Colorado
2014 Eagle 30.5 RLS
2015 Dodge 3500HD SRW 6.7L
Click on my profile for Mods and notes