“Site Pages” library was created automatically, after i access my site collection using SP Desinger

I have an Enterprise wiki site collection inside my SharePoint enterprise server 2013. Today I accessed the site collection using SharePoint Designer. I just navigated through the site collection content, without adding or removing any component. Then I closed SharePoint Designer.

After that, I noticed that there is a new library named "Site Pages" have been automatically created as follows:

What could cause this library to be created and why would accessing my site collection using SharePoint Designer force this library to get created?

second question ,,is there a way to know exactly what causes this library to be created ?