I switch browsers somewhat often, and I'd like to have all my bookmarks in the same place (not to mention the terror of migrating all bookmarks when getting new laptop). That's the main reason.

And of course there are little reasons such as:

  * native bookmark doesn't give me preview or little snippet of what the link is about.

  * I want to replay Freddie Mercury songs on YouTube in loop.

  * It's always a pain to download photos of myself, tagged by family on Facebook.