To put it simply, Krumo is a replacement for print_r() and var_dump(). By definition Krumo is a debugging tool (initially for PHP4/PHP5, now for PHP5 only), which displays structured information about any PHP variable.
A lot of developers use print_r() and var_dump() in the means of debugging tools. Although they were intended to present human readble information about a variable, we can all agree that in general they are not. Krumo is an alternative: it does the same job, but it presents the information beautified using CSS and DHTML.
Except the collapsible DHTML tree built around the structure of the dumped PHP variable, and the improved by the CSS looks, Krumo offers additional useful features.