It makes sense to collect everything possible, then discard what you don't need later, because when conducting an investigation you don't know in advance which data are of interest.
If your communications are intercepted, stored, but then never looked at, and eventually deleted - this is functionally equivalent to having never been collected at all.
If your communications are intercepted, stored, but then never looked at, and eventually deleted - this is functionally equivalent to having never been collected at all.