"Why are you tracking all the users and generating a huge 'haystack' of noisy data when you could track the 'needle' instead?"
A possible scenario is that Joe Bloggs is arrested for say drug dealing. They find that Joe Bloggs has 3 mobile phones and 1 ADSL connection. They contact those providers for a list of domains/times/IPs which messaging services were accessed. They use those details to make a request to the messaging providers for access to their messages to see who he contacted.
I imagine this would be cheaper/quicker than trying to forensically examine the devices. It won't catch any savvy criminals but that also wouldn't have been the case with phone records if they used pay phones or "burners".
The other, less positive explanation for why they think this useful is that they really are interested in monitoring the haystack to see how many people are visiting the sites of certain campaigns or political parties.