If you travel a lot, Toilet finder.
Edit: and not an app, but a website: Pairdrop - really useful for cross-platform file sharing, especially when you just need to email to colleagues something you snapped with your personal phone, but yoe have overly tight IT systems in place at work that stop you from connecting your personal phone to your email or OneDrive.
I come from Poland and yes, totally. When I started school, and missed lessons because I was sick or whatever, I could just take the phone book and find the surname of the classmate I wanted to get notes or homework from. If there were a few surnames on the list and I didn’t know their father (it was always the man of the house who was listed) first name, I could just go by who appeared to live closest to the school. Or just start calling all the numbers until I got the right one.