Not all companies are created equal. Likewise, the same job in different industries can pay significantly different amounts. Being aware of these disparities -- and finding opportunities because of them -- can provide very valid reasons to change jobs.
Some industries feature a mix of large- and small-scale companies vying for similar work. For example, you may find that a smaller contractor than the one you work for doesn't have the consistent work of its larger counterpart, but makes up for it with an exclusivity that allows it to charge higher rates. Conversely, you may find that a larger employer can provide more steady pay to its sales force than a smaller competitor, thanks to its broader market reach. In situations like these, employees in identical positions at different companies may have vastly different pay scales and growth opportunities.
In a similar fashion, your job skills may be much more valuable in one industry than another. A marketing director for a small nonprofit, for example, may make significantly less than a marketing director at an industrial manufacturer. Being aware of what your counterparts in other industries make can alert you to these disparities -- the difference in pay or opportunity may be enough to justify a job change.