Bathtubs are used mainly for soaking in and getting clean. Some folks have found additional uses for them like brewing gin or hanging out with the baker and the candlestick maker. They've also been used to make great discoveries. Eureka.
Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.
— Archimedes of Syracuse
What kind of discovery did you think I meant? Maybe it was the nasty kind of discovery made by the occasional victim-of-the-week on SPN. Perhaps the discovery would be that if the water in the bathtub looks like industrial sludge, maybe you should pass on the bath for the evening….and leave the house. At least grab the salt. Sam Winchester is too busy chasing his brother to get to you in time to pull you out of the clutches of some vengeful spirit or tentacled critter. Don't let this happen to you.
You should probably also avoid granting interviews to the opposition while soaking in oatmeal.
One thing you do not have to worry about though is going down the drain with the bath water. Mr. Rogers assures us that this is so.
Some find spending time in the bathtub to be restorative. Most meatsuits are appreciative.
A warm bath is always nice after you've been roughed up by the mob.
Others find that a cold bath is the way to restore a sense of wellbeing.
When assisting others, try to pick a large enough bathtub to cram them in. Remember Archimedes and the water displacement? You might also want to invest in a bathmat. Maybe a mop. No one wants a broken neck from a careless slip on a wet floor.
In the event of a loss of water pressure, make sure your pal the sargeant is available to haul water. An audience is up to you.
Some find that bathing is too much of a hassle, especially when they are wandering (but not lost).
Or fighting ringwraiths.
Some days, finding a water resistant covering for your mechanical arm can put a damper on any enthusiasm for bathing. I hear it's a good cure for a Hydra hangover though so it might be worth the risk of a few short circuits.
A few bubbles and a pal who's cute and yellow and chubby can make all the difference in the world so what are you waiting for? Rub-a-dub-dub.
Caps from screencapped.net, Home of the Nutty, Wikipedia and wallpapershdtv.com