Why does a person need such a feeling as excitement, because it only interferes?

Why does it only interfere? Excitement is excitement, it is an upsurge of energy, mobilization.

A public speaking WITHOUT excitement may be technological, but excitement ignites a spark - in you and in the audience. Without excitement, you may be simply bored with what you say or offer to people. And then they too - either too bored, or, in general, not: you are a professional, and technically you do everything right - but you, you! - you can clearly see the difference with what happens when you worry and your awe draws the audience into LIFE here-and-now.

When I was studying to be a doctor, I was wildly worried before exams, and ended up with red diploma. And when years later I passed my first training as a psychologist - yes, I passed all exams for 4 and 5, but I was not so worried about the results and the future that I was not going to write and defend my diploma. Then there were other studies, I have a diploma, but I mean that my relaxation was just a real complete absence of excitement. And mobilization for the important final stage!

So excitement is good. It is just important to learn how to regulate its degree. It's like having sex. Sex is joy and happiness, but sex with everything that moves - um ... It rather worsens the quality of life)

Excitement is a transformed instinct for self-preservation. When people go to a report to a senior in rank, or to speak to the public, the same primate lives in them, fearing the leader of the pack, which can drag in, or the same pack, which can throw stones or ostracize. Actually, that's why excitement is more typical for someone, to a lesser extent.