It's absolutely true that Windows 10 Mobile includes a very nice 'timer' inside the 'Alarms & Clock' application. So why would you need another UWP application that does the same thing? Well, to be honest, you wouldn't, but Timer ++ gets some props for being prettier and more intuitive.
Meet the new version for Windows 10! Incredibly easy and enjoyable timer! Forget about the awkward controls! Choose a time, using a circular slider!
Disabling the lock screen
Keeping recent values
Work in background
That's about it, really. The combination of setting the timer err... time with your finger, spinning the dial round, with the remembering of all your past timers in a scrolling carousel at the bottom, makes for a winning combination, though given the use of improved cosmetics over Windows 10's own timers, I'd have liked to have seen a choice of themes and colours as well.
Here's Timer ++ UWP in action:
Just dial up the delay! Bit by bit, the most common timer delays used are stored in the controls at the bottom of the UI; (right) As and when the timer expires, you get notified by a named toast, even if you're in a different application.
It's a bit bare bones at the moment, with just a lockscreen override, should you want the UI kept with the screen on for the whole of the timer's duration (e.g. watching it with one eye while doing something else that's time-critical), vibration, and choosing the sound to play at the timer's expiration.