Uncapping your framerate is a common way to minimize input lag. Even though your monitor's refresh rate may be constrained to 60Hz, for instance, frames can actually be delivered more frequently than 60 FPS when uncapped. This may sound counterintuitive at first, but it becomes clear once you understand the difference between a monitor's refresh rate and a game's framerate.
Every time a frame is generated by a GPU, this frame will be placed in a buffer that the display can then scan in and display on your screen. This scanout process reads in and displays the frame one pixel at a time, from left to right, then top to bottom. It happens very quickly, typically over the course of 16.6ms on a 60Hz display. In the absence of V-SYNC, it is possible that the next frame generated by the GPU will be swapped into this buffer while the display is still mid-scanout. This allows multiple frames to be displayed in a single screen refresh, which causes screen tearing, as seen in the bottom left of the diagram.
While seeing partial frames as soon as they're swapped into the buffer is a natural advantage of V-SYNC off, reducing input delay, the interesting part happens when your framerate exceeds your display's refresh rate. In this case, it is actually possible that several frames are included in each scanout.
In this extreme example below, the display refreshes at 144Hz, but because frames are generated at 1,000FPS, we are actually getting many frames in a single scanout. This essentially "defeats" your monitor's scanout, allowing us to display more than 60 FPS on a 60Hz display, albeit only in partial slices.
Since we are seeing frames as early as the display will possibly allow, increasing the framerate here will actually improve responsiveness beyond what you would see with V-SYNC off at lower framerates. How much you will notice this varies from person to person, and you need to deal with severe screen tearing to get this benefit. Thus, you need to decide for yourself if this enhancement is worth the tradeoffs, perhaps even on a per-game basis. For example, this technique is common practice in fast-paced and highly competitive games, such as CS: GO, where responsiveness is paramount over visual quality, but it wouldn't offer an appreciable difference in slower, turn-based games.
This information comes from a Blur Busters article describing G-SYNC, but it also includes some really informative sections on V-SYNC, frame limiters, etc.
I encourage you to read more for yourself here: http://blurbusters.com/gsync101