you should try searching before you ask. They need them due to the fact that the oil and air they contain expands and contracts with temprature changes. This would cause pressure build ups and blow seals with out a breather,
As you drive, the gears heat up which expands the air in the housing. The diff breather allows the pressure to vent out there instead of blowing diff oil past the axle seals.
Why it comes up in the 4WD community though is the opposite. If you've got the diffs nice and hot, then cool them really fast by entering water, it sucks air (and therefore water in this case) in to fill the vacuum which spoils your diff oil.
The FIRST mod to ANY truck that is going to be 'wheeled is extended diff breathers. On the 95 and newer trucks and 4Runners, the front is already extended from the factory. It's really easy to do, will cost maybe $20...
Gears heat up the oil, it expands.
Water crossings cool everything fast, time sitting cools slow, etc. oil shrinks.
Changes in volume of oil and corresponding room for air in the diffs is going to push/pull air/oil/crap from somewhere- you want it coming in/out off the top of the diff through a breather up in the engine bay or the seals?