Suppose for instance you wanted a static blog. Blog engines like wordpress (or django or flask for python setups) generate parts of the site on the fly as users visit. For instance, you might generate the most recent 10 posts in some list somewhere. This requires some (often not much, but some) additional time from the server as these are generated on the fly.

A static website generator like this allows you to make a blog (for example) and update content like most-recent-10-post lists (also for example) only when you actually add a new post. Thus no additional time is required on the fly.

I might add that there is an extension for flask (which I normally think of as dynamic) called Frozen-Flask which also allows a flask-built site to become a static site. This probably exists for django as well, but I'm less familiar with django than flask.