These provide the power necessary to have your microphone work properly, providing a full-throated signal to your recorder or computer. The problem is finding one that will meet all your possible uses-- connecting to USB for your computer, or connecting to audio jacks on your computer's audio in, or connecting to a mini-recorder!
Well, I found one! And it's from a brand I trust-- Behringer. If you need phantom power, this is the way to go. For the price, being able to output in a variety of ways is a real bonus! And the mic power supply is noiseless!
The Behringer BEHRINGER Audio Interface, 1x XLR/TRS 1x 1/4" 2X RCA USB, Black, 1-Channel (UM2), is a highly flexible tool. It supports one XLR input, two phono inputs (for instruments), and outputs to both USB and RCA jacks. This means with the right adapters you can output to just about any device, unlike most phantom power supplies. Highly recommended!