Planning is important to managers because the whole point of management is to allow a business to operate more efficiently and to be more able to achieve its goals. If there is no planning, managers cannot do these things.
In order to help a business achieve its goals, managers need to be able to spell out where the firm wishes to "go" and how it wishes to get there. They must decide, for example, whether and how the firm wishes to expand. They must decide how that expansion could best be carried out.
If there is no planning, decisions will be taken each day based simply on immediate needs. This will not allow the company to move forward in an organized and purposeful way towards its future goals. Such a firm is unlikely to succeed because it would have no long range plan.
For these reasons, managers need to plan if their firms are to succeed in the long term.