Why are there many different cell types in the human body if they all contain identical dna?

Well, DNA is just a big information store house. Every cell in the body contains all the information it needs to perform ANY function in the body. However, every cell does not perform the same job. We need different cells that specialize in muscle formation, brain signals, fingernails, ECT. It's really fascinating if you think about it. Each cell as smart as its neighbor but not too proud or selfish to simply do its own jobs. Hope this helps!