There is nothing more time consuming that trying to clean up a virus or install a large application onto an older machine. If such endeavor forces you to reboot the machine a few times, the time and effort to work on these old timers can add up to real dollars.
It might seem like if you have a good functional machine you would like to continue to use it to save on investing on new equipment. However it is simply not worth it. Fast machines can easily cut the trouble shooting time drastically, which means after a few hours, the cost of new machine can pay for itself.
This also goes for servers. When a troublesome server that is dated needs to be rebooted a few times during upgrades or maybe during a major issue, such machines boot up can literally eat up precious IT consulting time. It is not unfeasible to see a server take 20 minutes to reboot, so if there is a situation where 4 – 5 reboots are needed, this can lead to over an hour of just waiting for a server to reboot.
If reboot is that slow it is almost a certainty that working on them and trying to move large files or run various installs will also take longer. It is also pretty easy to say older computer have more issues in general which compound the problems.
With cost of hardware so low, having a fast machine always makes sense. For the daily user and for the IT Consultants that need to support it.