The inventor mode is a very important part of the game because without it, many things take very long to build. That can be very exhausting and may also be a disadvantage in multiplayer games as speed is an important factor in winning a game there.
The inventor mode enables you to create you own invention out of bricks which you can place in singleplayer-, multiplayer- and sandbox-games like any other block. With that the game turns into a strategy game where you can decide which buildings or weapons you can use. Invent a new, more efficient weapon or more powerful defences - the better your ideas, the better are your chances in winning a multiplayer game (or mastering a challenge in singleplayer).
With the inventor mode another big part of the game is implemented. But that's not the end - there are many things that will come in future updates. These include different maps and game-modes, more blocks, an improved economy system in single- and multiplayer, graphics updates,...
Hope you like it, tell me which feature you'd want to see in future updates.