If speakers began simply reciting essays in front of live audiences, no matter how fascinating the topic, they'd end up with a lot of sleepy listeners. That's because speeches are written to be spoken aloud while most essays are not.
For both an essay and a speech, a writer must consider their audience. If you're writing an essay on a scientific discovery to be published in a scientific journal read by scientists, you probably won't need to include a section that explains the technical terms used. For publishing on other types of platforms, an essay writer must assume their audience knows nothing about the subject and explain it in great detail.
For a speech, considering the audience goes even farther. No, you won't have to explain your technical scientific terms when giving a speech to scientists in the field of the discovery you're speaking about, but you do need to keep them engaged. Tone of voice, speech patterns, and physical deportment are all elements to consider when delivering a speech, and the speechwriter must consider the ways in which all of those elements will be used, as well.
In an essay, points can be emphasized using bold text, italics, and underlining. Thoughts can be organized into subtitles and sections, each of which can help make clear to a reader how the information they're being given has been structured.
A speechwriter must consider how to emphasize certain points verbally and how to transition between thoughts naturally but clearly in a way the audience will be able to understand. A good speech captures an audience's attention and holds it during a set amount of time while an essay reader can take their time, skip through points, and reread portions that are unclear on their first reading.
For an essay, sources of information to back the writer's argument or illustrate a point are often used. Whenever a writer uses a source in their written work, they must include a citation and end with a bibliography of the works they've cited. Making your source clear by properly citing them in an essay is important to avoid plagiarism, and the rules around presenting those sources are strict and must be followed closely.
Speechwriters also sometimes use outside sources of information, but a speaker won't keep an audience engaged by ending with a spoken list of citations. Instead, a speaker must attribute any outside work or statement to its source verbally, letting an audience know who the source is in a way that naturally flows throughout their speech.