This is a good question! :)
From an administrative perspective, if you are thinking about the Magoosh Score Guarantee, you will need to watch the explanation videos for at least every question you answer incorrectly to meet the requirements. So if that is something that applies to you, make sure to do at least that.
From a learning perspective, you should always watch the explanation video! Seeing the problem explained aloud may help you to gain a deeper understanding of the prompt or the process. You also may find a different approach than the one you took, which means you'll pick up new tips or tricks to solve questions efficiently. Thinking repeatedly through processes also helps you to recognize patterns you didn't see before. Finally, the video tends to do a more thorough job walking through what the text explanation captures in a succinct form.
Ultimately you will make the choice depending on how much time you have and how confident you felt in your approach, but more information is always better! :)