It's to show up inflammation.
Did they say what your ESR was? It's a number and the higher the number over norm (based on your age and sex) the more the indication of inflamation.
Men have a lower norm than women and it raises with your age.
Roughly...
Men under 50 years old: less than 15 mm/hr
Men over 50 years old: less than 20 mm/hr
Women under 50 years old: less than 20 mm/hr
Women over 50 years old: less than 30 mm/hr
I have a raised ESR as I get a lot of swollen joints with arthritis.
Some more info here...
http://www.labtestsonline.org/understanding/an alytes/esr/test.html
The results should be taken in context though as some people with severe inflammation in illnesses such as lupus can have ESR rates in the hundreds.