Wow - whole big review papers have been written on this subject. If you need more info, go to pubmed.com, some should be free and downloadable.
To be able to see what genes are expressed under different conditions is an invaluable tool. It is relatively easy to screen to see, for instance, how cancer cells express different genes than their normal counterparts. Treat the cancerous cells with a drug, and you can see how their expression patterns change - do certain genes that were expressed at the wrong time turn off? Also, how does the drug affect expression in healthy cells?
You can also design experiments to investigate the changes in patterns of gene expression through time - for instance, through development of a fetus (don't get too upset, I don't mean human, most of what I've read use rats as a model).
I hope that helps! Again, if you need more than that, try searching for review articles on pubmed.com.