Let's start with a claim that a four-year-old processor is still very powerful and can be put to good use. Now let's say that for a given amount of money you can probably buy either one new, very strong machine or about 10 older but very cheap machines. We claim that with 10 old machines connected into a cluster, by deploying load balancing, you will be able to serve about five times more requests than with a single new machine.
Why is that? Generally the performance improvement on a new machine is marginal, while the price is much higher. Ten machines will do faster disk I/O than one single machine, even if the new disk is quite a bit faster. Yes, you have more administration overhead, but there is a chance that you will have it anyway, for in a short time the new machine you have just bought might not be able to handle the load. Then you will have to purchase more equipment and think about how to implement load balancing and web server filesystem distribution anyway.
Why are we so convinced? Look at the busiest services on the Internet: search engines, webmail servers, and the like—most of them use a clustering approach. You may not always notice it, because they hide the real implementation details behind proxy servers, but they do.