While we all used mixers the first five to ten years of podcasting, technology advances have made them cumbersome, and more helpful for musicians.
You use a mixer to adjust the volume levels of different inputs (people) so they are all the same volume level.
When Do I Need One?
When you have multiple people in the same room as you. Everyone should get their own microphone, and have their own channel on a device?
So I DO Need a Mixer?
No. Now you can buy portable recording devices like the Zoom Podtrak P4 or the Rodecaster Pro from Rode. These have built-in mixers AND they act as a recorder and a USB interface to tie into your computer for remote interviews. Technology has advanced over the years, and the best part is the joy that was "mix minus" using a mixer has been eliminated. The new devices just do the hard work of setting up a mix-minus for you.
Who Doesn't Need a Mixer?
If you are doing a solo show, and maybe the occasional remote interview. You can but a microphone like the Samson Q2U (see microphones under $100) which is USB and XLR, and record remote interviews using Squadcast.