Actively developed OSes generally try to keep pace with the latest technology developments and continually optimize the kernel and other parts of the OS to become better and faster. Nowadays, the Internet and networking in general are the hottest topics for system developers. Sometimes a simple OS upgrade to the latest stable version can save you an expensive hardware upgrade. Also, remember that when you buy new hardware, chances are that the latest software will make the most of it.
If a new product supports an old one by virtue of backward compatibility with previous products of the same family, you might not reap all the benefits of the new product's features. You might get almost the same functionality for much less money if you were to buy an older model of the same product.