Choosing http or https during manual installation of WordPress and database

When you manually install WordPress, somewhere during the process it asks you to select weather your site uses http or https. Why is this? What does it matter? How will your choice translate to changes/a different use of eg your database?
Bc when I installed WP I chose http bc at that time I didn't had an SSL yet. Then I created a few pages, posts and categories. The URLs of these started w/ http.
Then a while later I installed SSL certificate on that domain name and since then pages, posts…

Choosing http or https during manual installation of WordPress and database