If you print your ebooks for some reason rather than buy used copies, granted you have to convert to pdf for that.
Why prefer an epub...
They have flowed text, and ebook readers let you customize font and font size so that the book's text adapts to you rather than you adapting to a particular combination of publisher quirks and zoom necessary to fit in a particular device's or app's viewable size.
They're generally smaller files.
And if there's something about the epub you don't like, you can actually fix it.
If a book is available in both PDF and epub formats, it's a reader's prerogative to pick the one they want, and there are specific types of books where PDFs might be preferable. However, if there's no original pdf, what is the objective of converting an epub to pdf in order to read it on a computer? Whatever you think you're accomplishing, I think you're mistaken. You end up with a pdf that looks exactly like how a typical ereader would display the epub. Minus the flowed text and font customizability, and probably with poorer navigation as well.