I always take it, but it's a bit of personal history

I once got stuck in a shower room because the lock broke, and I've spent about 6 hours on something like 3 square meters without any way to contact the external world nor entertain myself. It was just me, bath, shower machine and the sink. I've been alone at the house, so I couldn't ask for help. Also, hot water pipes were being repaired, so I couldn't take a nice shower/bath while at it, either.

Since then, I always took a phone with me. Just in case, y'know.