Each has turnover as the denominator, but each has a different numerator. Arguably, the only margin we should be interested in is the net margin, which is calculated on the net profit. But each calculation is worth looking at in isolation, as each one can give an indication of how a business is progressing and can be valued.
Margins can give investors an idea of a company's competitive advantage.
However, high margins are not necessarily the be all and end all of stock analysis. They often invite competition as other companies are attracted to the industry. On the other hand, a company with low margins is not necessarily a poor company. They can be very profitable by selling large volumes of low margin products, just like a supermarket does. Looking at margins is just one tool in an investor's armoury when analysing companies.