The desire of the ITX form factor is usually it's incredibly small size. Switching to an ITX sized motherboard can save you a lot of space and allow you to pack your computer into a very small and space saving case. With the right motherboard choice, you can even do the switch without losing any performance or functionality compared to mATX or full ATX systems - except for a few slots. ITX boards only have one expansion slot, meaning you can't run anything more than a single graphics card. Although you can still put a really high performing graphics card in the system to make a great gaming rig, the absence of any additional slots means that you could never upgrade to a dual graphics card setup, or add any other expansion cards that may be needed.
SilverStone, looking to show that with careful case design and component placement you can still have a near ITX sized case for an mATX system, has recently released their SG09 case. The SG09 comes from a long line-up of small form factor cases in SilverStone's Sugo series, and with such a small size it's going to prove that you don't need to abandon the mATX form factor if you want to go smaller!