It is a sensible question, we don't tell to pile on the hate for such things.
Originally it was an expansion pack for a DS slot flash cart. It provided three/four functions
1) Rumble. Certain games, or hacks to others, allow for rumble in games. This is then a third party offering for such a thing.
2) RAM. Most were concerned with the DS browser getting RAM to work but there is plenty of DS homebrew that uses it. There is a piece of GBA homebrew that uses the RAM (the PCE CD emulation aspects of pce advance) but that was built for the earlier GBA era EZ3 flash cart.
3) GBA games. GBA games require faster memory than the DS slot provides and the DS struggles to emulate a GBA outright so that is not really an option either. Step in flash carts/expansion packs with a section of memory in the GBA slot to run these games. However in most cases said packs do not have any other means to manage themselves so they need a DS slot cart to do it, they do however end up cheaper than the standalone carts.
It will also function to allow you to write GBA games to and then have the DS games which read the GBA slot detect the game it wants, pokemon tries to read the save game as well so will need you to patch your DS game to have it work (cory1492's pokepatch being the thing you want).
In the case of the 3 in 1 the EZflash team open sourced the protocols and software and became the main cart on the market as a result.
So yes buy one and you can use your DS slot flash cart to flash GBA games to it and avoid buying an EZ4, everdrive x5 or something like that. Compatibility will be about the same as the EZ4, which is to say damn near everything
http://gbatemp.net/threads/buying-a-gba-flash-cart-in-2013.341203/page-18#post-4756995 . The main difference will be that some management tools include patches for the nes classic/famicom mini series, which if you have a DS you could use one of the emulators for it to have a better time.
The main downside will be that if you want to use it as a standalone cart on a GBA or not have your DS flash cart present that you can only run one game at a time, also if you buy the DS lite size version (far cheaper than the few remaining GBA slot ones still out there) then you will need to mod the case to have it fit in anything else with a GBA slot that is not a DS lite.
