B-roll is critically important to making excellent videos. Without it, your video is bound to be boring and severely limit what you can do in the editing room.
With good B-roll, you give yourself options when it comes to editing, and you can increase the visual interest in your film. Don't just tell the story through a video, show the story. If your video is a live event, show the location through detail shots, show what is being talked about, don't just focus your camera on a talking head and walk away.
Think about what your video will look like for someone watching it on mute. Will they stay interested? What will make them turn the sound on? If they want to know more about what they are seeing, they will want to hear and see your video. B-roll helps add interest and tell your story in a more concise and impactful way.
Record everything, give yourself or your editor options in post production.
Here's a quick video I recorded just for fun to show what you can do with B-roll footage. This whole video is B-roll, telling the story without sound, just clips of what happened and what it looked like up close.