Answer:
Metaphors make a speech more memorable because in dull or formal academic writing, a metaphor can stand out and will be remembered by the reader.
Step-by-step explanation:
Not all metaphors make speeches more memorable. But good metaphors do. Sometimes papers and other works of writing can get boring and dull, and it feels like you're eating the writing equivalent of untoasted bread. Spice things up a little with metaphors. But don't add too much or it will be unbearable, and seem like you're trying too hard.
A metaphor is defined as:
a figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable.
An example is:
The spotlight was the sun, shining down on me.
Now, the spotlight is not literally the sun but it has the same effect to the person narrating.
But don't use bad metaphors like:
Her face was bad like rotten milk.
It doesn't make sense because a million other things could be bad, and you don't need a metaphor for such a basic word.
Your reader isn't going to remember, "This quote shows that Jane Austen feels for her readers." Though they may remember the point you made. But they might remember something like this, "Lydia Bennet's cocky nature was like that of a baboon." Even though that metaphor doesn't really make sense, it's memorable for sure!
Summary tl;dr:
Metaphors are memorable because they stand out among bland or academic text. Just make sure they're memorable for the right reasons!