Well, the tab key was created to avoid repeatedly having to press space to create an indentation, but that was on typewritters, so either way, you can use spaces or tab key to indent text.

Now text editors can transform tab character ('\t') to desired amount of spaces, and I think the problem with using tab characters in code ('\t') is that system may have different configurations for the amount of space a tab char takes, making it look inconsistent on different systems, while space char is pretty much standard on every system.