A dashboard simplifies that otherwise complex data you have in your spreadsheet and transforms it into something visual that's easy to understand and, thus, utilize.
Needless to say, dashboards have a wide array of uses from budgeting or project management to marketing or sales reporting.
For a simple example, I used a dashboard to transform this spreadsheet of first-quarter expenses:
into this quick pie chart that shares a breakdown of where money was spent during January:
This example is relatively straightforward. But Excel has tons of capabilities to create as complex of a dashboard as you require.