The excerpt can be an important part of the content of your post. Not immediately apparent where it lives while editing a page and not always clear where it shows up, it’s a good habit to get into adding an excerpt to every post on your site.
It shows up in RSS feeds (even if you don’t know you have an RSS feed, you do–and people use it, even if you don’t want them to), other feed-type readers and content aggregators (this is a good way to spread your content), and even, for example, in the WordPress Dashboard. In other words, add your excerpts.
