Your Android's Clock does exactly what it has to do, tell you the time. You might even want to use it as an alarm Clock, a stopwatch or a time. However, at Google someone thinks that you can get much more out of it, so they've decided to develop Clock.
More attractive and accessible functions.
Yes, this one also tells the time like the rest of them, the thing is that the latter comes along with a much more intuitive and accessible interface, with greater integration into the operating system.
Why should I download Clock?
Well, basically because it's easier to easier and much more eye-catching than the classic clock, but also because you can configure more alarms or follow different time zones around the world.
In turn, if you use Android Wear devices, you can share with them all your alarms and settings that you had established on your phone.