Nothing ever goes exactly to plan. Life isn't like that. That's why all businesses need to have processes in place to both review and evaluate. Note review and evaluate. The two aren't the same - in fact they are very different.
A review is all about "are we still on course?" and "what do we need to do to get back on course?" Reviews chart progress - progress against your original plans. That means they need to be regular, typically monthly, and they need to focus on progress against milestones (all good plans have clear, regular milestones). Reviews are part of making plans actually work - the planning process doesn't end with the plan. The plan in the beginning – regular reviews are needed. It's a bit like a ship's captain taking readings or soundings on a journey - it’s to stay on course and to make any adjustments needed to keep on course and arrive at your destination on time (and in extreme cases to change course if there are storm clouds on the horizon).
An evaluation is different. It's not a review. It's not about staying on course but about taking stock of the journey and capturing the learning - to help decide whether next time we take a different route. Evaluations are occasional not frequent and they focus on the 'big picture' - was the strategy sensible, did we have the necessary resources, were our assumptions sound, what would we do differently if we had our time over again? Evaluations help you to learn and to improve - they help you when you are considering developing a new strategy for the business or when you are embarking on a major new project such as the introduction of a new product or entering a new market.