The agents apparently took records going back to 2013. Cohen was not a trial lawyer, but a legal representative who swung financial deals for Trump and worked to suppress embarrassing news like the Stormy Daniels tempest. As Trump's main "fixer", he could have very pertinent financial records that would be prosecutable under NY state law, if Trump were to pardon Cohen. So I can see why Mueller and Rosenstein did it this way--to make sure that Trump would continue to face investigation of his crooked past dealings even if he did manage to derail the federal investigation into his dealings with Russia.
Right now, Trump is bouncing off the walls, looking for any way to strike back. It is a particularly dangerous time, because he is going to launch some action in Syria that will likely attract a lot of eyeballs away from the Mueller and Cohen dramas. Russia, which is now the major foreign stakeholder in Syria, has been making threats of "dire consequences", if the US attacks Assad. Trump also has options for making mischief against Iran, North Korea, and China. Any distraction will help. Unfortunately, he is prone to taking actions without considering the consequences, so he could involve us in some serious military actions.