My first, my last, my everything.
Those lines from Barry White are important for storytellers. We tell new students, "Don't try to memorize the whole story. Memorize the first and last lines. That's a good start, but we need to go further than that. Before we memorize them, we have to do a good job at crafting them.
There are many roles that your first line(s) fulfill. They should set the scene and the mood. They need to make your audience want to hear more. They can convey who you are and where you are going to take the audience. They should engender trust from the listeners. Will they want to go on this journey with you?
Your last line, too, is crucial. It is the last thing the audience hears. It is one of the most important things they will remember. Make sure you say it clearly, with the intention that you want. And make sure you have clearly decided what that intention is.
The last line can wrap up the story. It can make everything clear; answer the question that was posed at the beginning or in the middle of the story. It can pull all the parts of the story together. It should give the audience a sense of closure and satisfaction. You can challenge the audience to think, or decide for themselves how things turn out, but make sure they have a solid basis for that, so you don't leave them hanging or angry and unfulfilled.
To sum it all up: Your "first", your "last" should be memorized, but like your "everything" in the middle, they should be especially well crafted.