You can create content in several ways:
1. From scratch
You just write what’s in your mind.
Note: this is not legal advice – please consult with a lawyer before using existing content.
2. From existing, by refactoring
For instance, you find an ebook with Creative Common rights, and create a derivative of it.
Why would anyone do that?
Let’s say it’s a Math book and you want to change the layout and want to keep only some part of it. For example, you want to keep 20 problems, with solutions, but you want each problem to be on its own page, the solution to be on the next page.
3. From existing, by adding to it
In the previous example, let’s say some problems in the original ebook don’t have solutions, just hints. The modified version could include the solutions created by you.
Some people like to do farming, and plant the seeds themselves. Others like to take an existing product and improve it.
Do you like to build from scratch or to modify existing content, for which you have the rights to modify it?