When a doctor wants to carry out a test, she will probably prick you with a needle, fill up several test tubes of your blood, label, package and send them to some centralised hospital laboratory. Technicians will then take the contents, perform the various biochemical analyses needed, write up the results and send back the documentation in a few weeks, perhaps longer if there's a backlog.
The process is slow and labour-intensive. What if you could reduce the whole business to a few minutes? What if, for the majority of ailments or questions, the doctor only needed a drop of your blood and could test you for viruses or cancers while you wait in her surgery? With a lab-on-a-chip, that is already possible.