For me its not necessarily a raw measure of just time saved.
I realized that the physical habit that we've all developed of pulling our phones, unlocking , checking time or messages etc is a really really annoying physical habit.
It's disruptive and awkward to do, interrupts conversations or basic interactions with things in your environment.
A two handed fumbling time check/message check becomes a no handed operation with a smart watch.
I don't actually check my messages any more often, in fact less, since the notification is actually much more obvious then phones chirping in a pocket and my decision about whether to handle the information now or later is just much more quick and fluent.