importance of using histogram equalization
The difference is that with your method, if you imagine the histogram, you're simply going to stretch it to span from 0 to 255, but its shape will be preserved. Histogram equalization not only stretches your histogram, but also tries to make it flat, so that you get an approximately even distribution of pixels of every shade of gray.

In terms of why one is better for certain applications than others, that's application specific.

Edit:

Here are some examples from Bruzed:

Contrast stretching (your method):



Histogram equalization:



As you can see, the bulk of the pixels in the original image was gray, represented by a large peak in the middle. When you do contrast stretching, the peak is still there, even though your darkest pixel is now black and you brightest pixel is now white. By contrast (pun intended), using histogram stretching you get a much flatter histogram response. This actually increases the overall contrast of the image.