What would happen without a marketing plan?
Answers
Let me change your question: Why do you need a map when you take a trip? What would happen if you don't have a map?
Some trips are long and complicated, through territory you've never been before. These trips are made much easier when you use a map or GPS. You will probably have measurable goals (Rochester by lunch, Cleveland by dinner, hotel in Toledo) That way, if you end up in Dayton you can make the necessary adjustments To arrive at your hotel in Toledo.
Other trips are short and simple and repeated so often that you don’t need a formal map, just a mental one.
Marketing plans are exactly the same. A large, complicated launch of a new product or attempt to find a new market for an existing product require a well-considered, formal marketing plan with interim measurable goals for awareness, attitudes and sales.
In contrast, a newer version of an old product, or an introduction of a new product that compliments an existing product may not require a formalized marketing plan. The same old marketing strategy will apply.
The important similarity is that when taking a trip or selling a product you know where you are starting, where you are going and why you are going there. If you don't you will end up horribly lost in the wilderness with no idea where you are or how to get back. Unless you are very lucky you will suffer a long, slow, agonizing death from starvation.