If I'm walking or hiking, the goal isn't to exercise, it's to get out, destress, explore a new area, or connect with nature. Those times I don't start a timer because it doesn't matter how much exercise it is. The goal there is to release and let go, any exercise that happens is a happy happenstance.
However, looking at the clock for a reference point of not wanting to push too hard at the beginning can be helpful. For instance if you are doing a 20 min exercise rotating between 3 different movements, looking at the clock say at the half way mark can provide some feed back on how the last 10min are going to go. how do you feel? Can you push more, are you dragging and need to slow the pace a little or are you feeling like you can sustain the current pace for the last 10min? These are the type of thoughts i want to evaluate when looking at the clock!