For the most part, planners are in the purview of organized, type-A go-getters (i.e. crazy people), but more laid back folks (i.e. slackers) can benefit from some form of planning too. Even if you're just using a monthly wall calendar, you can reap the rewards from having a hub to document the things you can’t financially, emotionally, or legally afford to forget.
Bills
Bills are one thing we all collectively have in common. No matter how not busy you are, you have bills to pay every month. With a planner, you can write down the days they're due and for what amount, so you never forget to pay them again. This is still a useful thing to do even if you’re signed up for automatic payments. Sadly, technology fails us at times, so you need to double check that your bills are being paid automatically every month. Otherwise, you could find yourself on the phone with a very angry collections agency for three months overdue charges on your car payment.
Appointments
You might not have a medical appointment every single week, but for the love of God, pop by your doctor and dentist from time to time. Nowadays, those places get really peeved when you miss an appointment, so it’s in your wallet's best interest to attend. With a planner, you can write down those appointments when you make them to prevent forgetting later on. Plus, it helps you remember to ask off work for that hour or so.
Scary Dates
If you have a job that tends to assign projects, you probably need to know (Editor's note: you DEFINITELY need to know) the dates they're expected to be completed. If you're a library or rental person, remembering due dates is all that's standing between your wallet and a late fee. Make a bare minimum effort and write these scary dates down.
Fun Dates
More important than all of that responsibility stuff is the long list of fun dates you actually want to remember. Movie releases, weekend trips, your mom’s birthday, and other similar things need to be written down too. Planners can help you with bills and meetings, but life is meant to be fun—fun plans are what you need the most.