A daily planner app is a tool that holds your day in one place: the tasks you mean to do, when you'll do them, and often the habits you're trying to keep. That's the whole definition. Everything else is flavor.
Whether you need one is a better question, and the honest answer is: not necessarily. If your days are simple and your memory is happy, a notes app or a paper pad is plenty. Planner apps earn their place when three things pile up at once: more commitments than your head wants to hold, recurring things that punish you for forgetting (bills, meds, birthdays), and the creeping feeling that your day happens to you rather than with you.
What should a daily planner app actually do?
At minimum, four jobs. Capture quickly, because a planner you can't add to in five seconds will stop being used. Show today clearly, one screen that answers 'what now?' without archaeology. Handle repetition, since most of life is recurring. And forgive you, because you will miss days, and what the app does on the day after determines whether you're still using it in March.
That last one is the quiet differentiator. Behavior research calls the failure mode the abstinence violation effect: when one missed day is framed as broken, people don't resume, they quit. Streak counters that reset to zero are this failure, shipped as a feature.
How do I choose one?
Ask five questions. Where does my data live, and can I export all of it? Does it work offline? What happens when I miss a day? Can it hold my whole life (meals, money, people) or just work tasks? And does the free version stay usable, or is it a two-week ad for the paid one?
I build TaskPlannera, so weigh my bias accordingly, but those five questions are also exactly why it exists: local-first data you can export in one tap, offline as the default, streaks that sleep instead of dying, twenty-plus life sections, and a core that's free forever. If another tool answers those questions better for you, use that one. The planner that helps is the one that's still open next month.