Dry Airlock Question

[mpcluever]

I got a 200L variable capacity fermenter and it comes with a dry airlock.

http://www.midwestsupplies.com/plastic-airlock-for-enoitalia-variable-capacity-lid.html

It uses marbles to allow air to blow by and then fall back in place, sealing the hole. I get that part. Why does it have TWO marbles such that it allows air to go INTO the fermenter? There's one up top that allows air to go up and around the sides of the airlock into the fermenter. What am I missing here?

 

[malkore]

To avoid a potential vacuum from forming. I toured a very small distillery once and their mash tun had a faulty valve which didn't allow air inside as it cooled. The result was a 15BBL mash tun implosion. Luckily the manufacturing defect was under warranty so they got a brand new mash tun.