Safari can do some pretty useful stuff with bookmark folders. For example, if you have a folder in your Favorites Bar at the top of the window, hold ⌘ (Command) when you click it to open all of that folder’s bookmarks in a group of tabs. To create a new folder in your Favorites Bar, right-click it or use Bookmarks > Show All Bookmarks and use the buttons around the window there.
This can be useful if you have a group of sites you typically check in the morning, or sites you open when starting a certain kind of work; having Safari open them all at once for you is convenient, and it saves some time, too.
If you want a bookmarks folder to always open every bookmark in a group of tabs when you single-click it (note: this is only possible for bookmark folders in the Bookmarks Bar):
go to Bookmarks > Show All Bookmarks
click the Bookmarks Bar section in the sidebar
in your list of bookmarks in the center of the window, toggle the Auto-Click option next to a folder
Now, in the Bookmarks Bar, the disclosure triangle next to the bookmark folder’s name will change to a box, denoting that it will open all of its bookmarks in a group of tabs at once.
Lastly, in Safari’s Preferences > General pane, you can change the “New windows open with” option to any of your bookmark folders. This will open a group of tabs for that folder every time you start Safari or use ⌘-N to create a new window.