It would be neat if we cold create global, client-editable "settings", sort of like how Collections work now.
Where Collections are different groups of content, settings would be a collection of FIELDS, editable from a single page, which can then be used throughout the entire site.
An example might be a progress bar/thermometer that the client wants to update. There's no real solution to let the client do that without making very poor use of a Collection. Being able to add a "number" field as a site setting would let the client update that value, and the page could pull that value to fill the meter properly.
Another example might be setting an on/off toggle up with a text field to enable/disable an alert at the top of the page. You only need one of these, so while you could use a Collection for this as well, it's waste and counterintuitive for the client.