It's called a water-prime and is required where there is a potential for the water present in the trap in normal use, evaporates due to a lack of use, IE.vacancy.

These are also found in floor drains.

The water in the trap serves the function of preventing the sewer gasses from migrating to fresh air.

Trap prime.
As mentioned above, used to keep the basin/tub/shower/floor drain trap water seals intact. Might be needed in arid areas with high heat, low humidity, high winds combined with marginal venting systems situations emptying traps, or combinations of similar conditions.