significance of gram positive and gram negative?

Answer 1

it helps u to know which antibiotic to use or think about even before culture & sensitivity of bacteria. as antibiotics are known to affect either gram positive or negative or both (spectrum of antibiotic)

Answer 2

It's an empiric way of classifying bacteria, depending on whether they retain a crystal violet stain after being fixed with a dilute iodine solution and decolorized with acetone.
The composition of the bacterial cell wall results on whether it will retain the purple crystal violet stain (Gram positive) or the red counter stain of safranin (Gram negative).