Service planning should be like a strategic front-end (at the individual service level) that leads into the budget cycle.

It should make managers think broadly , within their immediate service/departmental bounds, about what they are doing, why and how before they head into resources, bidding and budget mode.

But it shouldn’t be too complex or onerous. This means having clearly articulated and well-understood (and differentiated) service planning and service review frameworks in place.
Like all organisation planning processes, it must have clear links with other organisational plans and processes (especially the annual budget cycle).
And, as always, the litmus test for planning and review processes is do they have the capacity to change/influence priority-setting and resource allocation?