The interior of the axle contains differential and bearings but also oil and air. Temperature changes in the axle are normal, whether heating due to friction or cooling when the axle is in water. Thus the volume in the axle changes (substances expand when heated and contract when cooled). Therefore, pressure equalisation is necessary, because otherwise the axles and seals have to withstand the pressure differences - resulting in a high load and unnecessary wear.
Therefore axles have an opening for pressure equalisation. But not only air but also unintentionally water and dirt can flow through such opening and penetrate the axle.
Example: If you are traveling with your vehicle and the axle has warmed up, the air escapes through the opening and leads to the desired pressure equalisation. Now you cross a water stream, the axle cools abruptly, the air inside contracts and causes suction through the opening. Now water is drawn instead of the desired air because the axle is under water.
To avoid this, we recommend raising the axle venting. So always, even when driving through water, air will be sucked as desired.