What is the use of mini mac ?

To add to Phil0124's info, it's also a computer for those who do not need, or want to pay for a built-in monitor. I use a high-end EIZO monitor for color and graphics work. The display panels included with an iMac or iMac Pro are pretty, but they aren't at the level of EIZO. Especially for calibration.

So for me, I really only have two choices when I'm looking for a new Mac. Either the new Mac Pro (way out of my price range), or a Mini. I do wish there was another choice in between that doesn't include a monitor, but Apple doesn't have one. You have to jump all the way to a minimum of 6 grand for a Mac Pro, which easily gets up to 10 grand without adding much. You can get a fairly well appointed Mini for just over 2 grand.

The Mac Mini is a compact computer that you can use everyday just like any other Mac. It comes without a display, keyboard or mouse. which allows you to use a verity of different displays, keyboards and mice. I moved to the Mac Mini's way back in 2010 after being disappointed by a couple of iMac's with display and graphics issues. Currently I have 4 different Mac Mini's, 2 are setup as our main desktop work stations, 1 is setup as a desktop for the grandkids and the last one is setup as a media server.

The same as any Mac or computer really.

The Mac Mini is just a small form factor computer. You can do anything you do on a iMac, or MacBook Pro laptop.

Its a cheaper alternative to the other Macs.