http://mywebsite.com/feed/podcast/ is a special podcast only feed PowerPress provides you. You don't have to use it. You can use as many categories for podcasting as you want, they are separate additional feeds provided by WordPress for each category. Just mark the post in the category and you're all set.
Just make sure you turn on Podcasting for that category. Go to PowerPress Settings, and at the bottom there's a section titled "Advanced Options", make sure "Category Podcasting" is checked.
The podcast only feed added by PowerPress is for folks who don't want to worry about selecting the correct category every time. Any post they add an episode to will appear in that feed.
Someone who uses category podcasting is usually an advanced podcaster who wants to separate 2 or more podcasts but maintain the same site brand/theme.