A blow off tube replaces most of the pieces of a three piece airlock. Airlocks can clog with krausen. The resulting pressure blows the airlock out or the bucket lid off spewing beer over adjacent walls and even the ceiling.
Picture is a blowoff tube attached to the center post of the three piece airlock. The end of the tube goes down into a small bucket with some sanitizer to keep crawlies from going up the tube into the fermentor. If this bucket is higher than the fermentor there is risk of the sanitizer being sucked back into the fermentor.
WY 3068 starts very fast, produces a very high krausen. The only time I use a blow assembly is with this yeast. There are probably some other yeasts with the same characteristics though.