These units are extensively used with professional and semi-professional PA syatems and in sound recording studios.
The direct box unit performs level matching, balancing, and either active buffering or passive impedance matching or bridging to minimize noise, distortion, and ground loops when playing any electronic musical instrument.
DI units are not pre amps or mic signal amplifiers. They are passive devices. All it does is give you better control of the input sign to prevent clipping of signals. DI units convert one type of signal to another. That said, there are a few expensive boxes that are supposed to work both as a DI unit and as a pre-amp for line level signal. They can not be used to manage mic level signal whether balanced or unbalanced.
DI boxes are 1/4 inch inputs and XLR and 1/4 inch outputs. When they say preamp/DI box, what they mean is that they can change the bass, treble, mids and add sound effects to the hot signal and then send it to the mixer at mic level or line level for a Guitar amp.
The most common application of a direct box is when connecting an electronic guitar or keyboard or any similar electronics to a sound system. The DI box allows you to connect into a snake or existing mic lines and send the signal up to 700 or 800 feet away.
To know more about direct boxes and its uses, check out this link: guitar center direct box
This information is written for guitar enthusiasts but should not be used as technical information. Many terms and descriptions are simplified for educational reading. Thank you.