Flash is used in for variety of reasons including the following:
-- use as fill flash to bring out details in shadow area under bright sunlight situations
-- illuminate the person when person is standing with his/her back against bright lights
-- allow the camera to photograph a person while keeping other distracting area in the dark
-- "freeze motion" a person as a person may not stand still for minutes for sharp image to be taken
Why is using flash in anyway related to shutter speed?
Why will many want to use a slower shutter speed if not using flash?
There are in no way or another linked to each other imo.
Like wad Squid has mentoned, bascially its act as a fill, or a primary light source, to reduce contrast, lift shadows and to "freeze" ur subjects blah blah blah..
Flash photography in itself is an art and good flash photography can open up a whole new world of creativity. Its scientific in another sense as well (etc. inverse square of law, exposure variation due to flash to subject distance..)
I believe some read up in google will do u gd in the long run friend
There is lots of reason to use flash. For me i would even use flash when the sun is very bright or you can term it as harsh. I cannot wait for the sun to go away so have to fill flash to lighten the shadows. Even you can wait for the sun to go down you will get a longer shadow.