HeavyLoad does its darnedest to consume all the resources on your computer and strain it to its limits. Why would you want to do that? If you're a typical business or home user, you wouldn't. If you're a programmer, however, this freebie could spare you (and your customers) a great deal of pain and waste.
Very often, programming errors can "hide" from programmers due to copious system resources. A small memory leak might not be noticed. A buffer overrun can be harmless when memory is mostly empty. Then a client or user starts reporting errors on their lower-end or over-worked machines, and the developer is hard-pressed to duplicate them on their larger, more powerful, systems.
HeavyLoad helps locate these issues by basically sucking down all computer resources: running the CPU (including multiple cores) at maximum, creating a massive temporary file to strain read/write on the hard disk, and filling memory. If your program can keep performing under those constraints, it's pretty good. It's also a good way to find out "Minimum" and "Optimal" specifications.
HeavyLoad is obviously a program with fairly specialized uses, and it's not going to be run every day. However, it's freeware that performs a useful testing function, so it's worth checking out.