many telephone companies and internet service providers destroy
logs of customer activities when the information is no longer needed
for their business .Government in many countries proposed laws
requiring that such logs be kept longer for use for law enforcement
agencies. does these logs are useful for law enforcement
agencies if so how are they useful.

the logs are very usefull. i believe this website will answer most of
your questions about isp logs.

http://www.cippic.ca/en/faqs-resources/online-anonymity/

This is an essay question in a text book, I THINK the teacher requires
YOU to do the RESEARCH and find the ANSWER yourself. I have the same
essay question, and I'm doing research for it now.