Everything possible must be planned, from the application's structure to the user flow.
Thus, you can remotely monitor the application before you develop it. That shows you the correctness of the structure and allows you to see the missing parts, the excess parts, and the application in its entirety.
One of the keys to being a good developer is planning.
1- Your ideas about what and how to build become clear.
2- While the development is in progress, people can lose themselves and add unnecessary features to the application; if you have planned, you have already gotten rid of this.
3- The database structure begins to settle.
Designing a database is also a topic besides planning, but getting specific points before creating the database helps you design the database.
4- It is predetermined that you will send things that engage the user with your application, such as transactional emails; this increases your development speed and allows you to make the right decisions for your engagement with users.
5- You specify the pages.
I seem to hear; I add as many pages as I need; why should I plan this?
Nothing is as simple as it seems.
Sometimes we build a whole application on a single page; sometimes, we divide a small application into many pages. User experience, what the platform offers us, and other things influence these decisions. That's why it's crucial to plan even the pages ahead of time.
6- You create the user flow.
Creating the user flow lets you see the application as its user, which gives you a great starting point for editing and wireframing from a cleaner perspective. When you interact with some applications, you must have thought, why does this button lead me here? What should I do now? Planning the flow will reduce the bad UX experience.
7- You can start thinking about marketing from the very beginning.
The idea is one thing, development is another, but marketing is the most important. Even if you present your great idea with a great design and an app that works like a clock, if you can't market it, you can't sell it!
Since day one, I haven't done everything I wrote here at the planning stage. I didn't even do it in my first year. At the end of my 3-year learning journey, I started to gain this kind of working principle. More than 10+ of my side projects have taken their places in the web graveyard, and some of them that I am currently running will end up in the cemetery—the nature of this item.
Still, I have learned and continue to learn ways to reduce the number of projects to the cemetery.
So do not be afraid!
Before trying to do everything, I wrote here, focus on improving yourself. When you combine the experiences, I tried to convey with your own experience, you will get the maximum benefit.