Another way of asking this question is "What if I don't bother with project management?" The fact is, all projects demand some degree of attention to make them happen. Even the secretary needs to deliberately plan to set aside some time to clean out that filing cabinet, otherwise the job will never get done! Believe it or not exactly the same holds true no matter what the size of the project. Indeed, the bigger the project the more difficult it is to get it started.
But most projects also require some degree of coordination of resources, and unless this is carefully planned either things will be done in the wrong order or there will be constant conflict and crisis.
All of which will ultimately consume a lot more resources, time and money than necessary.