The reasoning is that there may be new information that helps identify conspirators or abettors. We all know the chances of finding smoking-gun type evidence on that phone was slim, but that doesn't mean you ignore possible leads when attempting to piece together how a crime was committed. Just because the shooter didn't intend to leave any evidence on the company-issued phone, doesn't mean that there wouldn't have been meaningful data on there that would advance the investigation and generate valuable leads. Think GPS info, or items that complete a timeline, etc.

Here's an example. Suppose they uncovered GPS info from the phone that showed that the shooter visited a particular location frequently in the days leading up to the attack. FBI checks that location out, and discovers that it is implicated in another ongoing investigation for money laundering. The other investigation wasn't tied to terrorism, but this thread creates a link and enables another source of support to be found and squashed.

My example is just an illustration of one of many possible ways the data on the phone could advance the case. It's absolutely worth pursuing.

To be clear, I'm 100% opposed to any attempts at government backdoors, and I sided with Apple when they refused the FBI's request initially, but that doesn't mean the Bureau should just shrug their shoulders and not do their job.