The latest web design trends dictate that your website should be modern, fast and easy to navigate.
While the content of a website may be well constructed and up to date, the design may have been left behind, mostly on websites that were built a long time ago, when the demands were not that high.
That could cause your visitors not spending enough time on your website trying to find what they are looking for. If they abandon it, th chances are they will never visit it again, depriving sales from your business and therefore, profit.
In this case, a website redesign is the only way to go.