I can see that there is no need as I can just select targets without minding time and horizon [within reason] and scheduler will take care of snapping them up over the days to come.
You understand!!!
My question is does scheduler use the horizon information setup in the ACP Observatory tab?
No, it uses the horizon constraint, along with the other constraints such as hour angle, air mass, Moon avoidance. etc. These are applied on a per-Observation basis, so even within a single Plan (the minimum schedulable unit) you can have different constraints on possible multiple Observations (the minimum runnable unit) within the same Plan. This may seem confusing, but as you learn to think "scheduler" it will become clearer.