As you can see from this side-by-side, the key differences boil down to: complexity vs. simplicity of the data. If you're working with plain text data, this can be done in either CSV or Excel format. If you're going to be working with data that is more complex - containing formulas, graphs, multiple tabs, etc. - this is when it's time to power up Excel.
Both formats are well-suited to various needs. CSV, because of its simplicity, is the preferred format for moving data
(importing and exporting). Excel is the powerhouse for data analysis and computing. Use both formats depending on your particular task!