why is classical music used in film? what impact does classical music have?

What impact it usually has is being past the copyright date.

Why shouldn't it be used? Perhaps if you're able to relate the music to what;s occurring on screen you'd realize the impact it has.

well, the music must enhance the emotion portryaed. Classical music is 'solid' has depth(usually) and doesnt go out of fashion!

Do you mean classical as in compositions by masters - Bach, Beethoven, etc. - or Music that sounds as if it was classical - just because it is played by a large orchestra?  People often confused the two . . 
And if a studio orchestra is playing Mozart, for example, the those arrangements often ARE still under copyright.  Ib been in the classical music business since 1973 - I know how to determine where the decision falls - and it changes often.

The copyright is expired.