It's important for grepping and sedding - the colors are escape codes and would mess with your regular expressions otherwise (you want to find foobar , but bar was blue so you got foo\e[1;34mbar\e[0m instead, and it doesn't match with foobar).
On the other hand, if you want colors displayed you can force grep/ack to display the color 'always' and then pipe the output into 'less -R'. The -R (or --RAW-CONTROL-CHARS) flag has less display output such that color escape sequences work as you might expect, coloring the output displayed by less.
