Each block has its own set of default attributes. When you add a block on the page, default attributes will be used. What if you need to have your own default styles. For example, you want to make all new added Headings with violet color or specific size.
For this, we have a special option to save our own default attributes.
Add styles to any block, then, click on the brush icon and select them to be default. Now, reload the page, and try to add a new block. You will see that it has updated styles.
Previously added blocks will have old attributes