the reason for also checking the locations where cellphone calls were made not only of possible suspects but also those of people the possible suspects called is simply because even if the alibis of possible suspects are airtight, there is always the possibility of either their relatives, their friends or hitmen they hired doing the dirty work of murdering a victim.
LE should do cell phone forensics of possible suspects as well as people that the possible suspects called recently. A recent example of using cell phone forensics shown below:
http://news.yahoo.com/body-found-search-missing-nursing-student-145114754.html