Adding Photos in Post to Media Library

I migrated my blog from blogger to WordPress and all of my posts have images in them. Is there any way to add them to the media library?

I couldn’t find any plugins to do this and the only thing i can think of is to manually download each image in each post to my computer and upload them to the media library and then replace every image in every post with the updated version – quite time consuming and error prone. Is there any advantage to having every image in the media library? Otherwise I would only do this for featured images needed – but still there are hundreds.

I’m not very familiar with how the media library works and best practices – am I missing something?

Thanks!