There's a lot you can do, if you get clever, with regards to education reform, climate change and energy development, immigration, LGBTQ protections, substance abuse problems, etc with executive actions. Legislation is usually better at baking in real change, but the operational control the President has is still very valuable.
That's not to say Bernie doesn't care about those things, I believe that he does, but I don't have confidence that he's organized enough or understands administrative politics enough to be nearly as effective. And the stuff that is the core of his campaign, like sweeping tax reform, single payer healthcare, etc, that stuff all requires actual congressional action