what is the use for fitting a fish eye ,dose it not narrow your field of view?

They're actually a very wide angle lens with significant distortion. The shots from them have a globe-like perspective, pinched down at the sides and bulging in the middle. This is because they gather light from a 180 degree field, though in most cases only the middle is projected onto the camera's sensor, which gives a sort of "wrap-around" look to the photo, that but there are ones that actually project their full 180 degree view onto the sensor, leaving a true "fisheye" view in the middle, with the rest of the image blacked out (think of the layout of a Japanese fllag).

When you see those security camera pics or videos that have that very wide angle distorted look, that's a fisheye lens at work.

Here's a couple pics from Wikipedia that display the two above examples:

Centre of fish-eye image projected onto the sensor.

Full fish-eye image projected onto the sensor:

Extreme wide angle and distortion

the latter being part of the fun