Lets start with the term 'benchmarking' in general, and look at its application to the IT world. When one preforms a "benchmark" on their computer, they test different aspects of it including its ability to run different animations at high speed. This result is recorded and a number (score) and
can be compared against others worldwide.
In the business world, benchmarking is the comparison against compeditors in reagards to different business-oriented implementations. What Do I mean by that?, simply put, a business compares itself to another business. This is usually used to determine how successful others have been in implementing change, what sort of effect its had on them in comparison to your business, levels of absenteeism/turnover to discern employee satisfaction - ultimately attributed to different implementations in this other business, comparing expenses, etc ..
A business benchmarks in order to determine how successful an implementation of change has been in relation to other business that have followed a similar route. One could also benchmark against previous results from past years of their business as a means of observing the development of their company as a result of change, and how this change has impacted specific elements.
Hopefully that makes sense, just reply back if theres any queries .. im sure others will offer insight aswell though