Handrails are normally installed where a potential hazard exists, such as a large drop at the edge of an elevated deck or where stepped access is required.
When installing handrails the two most important considerations are:-
The bottom rail acts as a barrier trapping leaves and other debris on the deck
A fixed rail also prevents proper cleaning of the deck surface.
With the rail fixed to the deck boards should access be required under the deck then the handrail would need to be dismantled first.
By keeping the bottom rail clear of the deck you are eliminating these three problems.