It's used when you find that the exposure automatically determined (auto, shutter priority or aperture priority etc. mode) by your camera is not what you want. The metering and setting algorithm in the camera is not 100% programmed to suit your tastes at all times and therefore do not always set the exposure you want.
In situations where there is a wide contrast between subject and the background, the camera may end up with an exposure that over-expose or under-expose the subject and you would then use -EV to tell the camera to step down to cut down the over-exposure or +EV to step up to allow more light when yoiu think the subject is under-exposed.
Of course, it's of less use if you shoot with full manual control since you decide all the settings yourself.
As a general rule of thumb, you overexpose (Ev+) when the backlight is strong (quite common) and underexpose (Ev-) when there is a strong light source on your subject and/or the background is large and dark (usually stage or night scenes).
As a general rule of thumb, you overexpose (Ev+) when the backlight is strong (quite common) and underexpose (Ev-) when there is a strong light source on your subject and/or the background is large and dark (usually stage or night scenes).