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.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10