What is the small tube for?

[c112112]

Hi folks, I installed a shut-off valve on the shower head earlier and always feel the water goes "somewhere" after the valve goes to off position.

Today I open the dry wall and found there is a small tube attached to shower pipe, what is that tube for? It looks like water goes from shower pipe through the small tube to the washer and drier room's drain.

Does that mean I cannot have the shower head shut-off valve?

The valve is as below:

Thank you very much!

 

[Geofd]

who ever piped that was using that tee as a trap primer every time the tub spout is used water fills the washing machine trap so the water in it wont evaporate its not anything I have ever seen …..I wouldn't use it if your washer drain ever gets clogged and your not doing laundry often it could be a mess
using those shut offs on the shower head can cause the hot/cold water to cross over causing water temp issues...….
yes all the stars have to line up for each to happen but that's my take on this post

 

[c112112]

Understood. Thank you very much!

Is it a good idea if I break the water-fill connection? Don't want to see water wasted very time I take a shower...