Yes. Yes you do, but you already have one if you have a smartphone.
I love a succinct answer. No point in dragging these things out.
When I decided to write an ebook, it did cross my mind that I was potentially excluding a large chunk of the reading public who didn't own an ebook reader like a Kindle device or a Kobo.
But actually, most people already do own one. If you have a smartphone like an iPhone or any Android phone that has apps, then you can get the Kindle app and use it as an ereader! Simples.