What is the purpose of cockroaches?

Based on the article I've read, it has two main significance: Ecological Significance and Significance in Medical Research.

Ecological Significance - cockroaches breakdown nutrients of decaying matters. They play a major role as decomposers too.

References:

http://yougotme.hubpages.com/hub/Why-Do-Cockroaches-Topple-Over-When-They-Die