In the equation E=mc2 why is the speed of light squared i mean what is its significance?

here the speed of light acts only as a constant of
proprtionality.....

1. look E and m are
inter-convertable.....and in such processes E is directly proportional to
m.......

2. now to replaceproportionality by equality we
must require a constant.....

3. morever in such processes a
very small amount of mass can be converted into a very very large amount of
energy.....therefore the constant heree will have a very large value....and when
calculated, its valuewere found to be equal to 90000000000000000 and its unit should be
in m/s that is exactly equal to the square of speed of
light.....