From what I have seen, it is like RAM vs HD usage. You bookmark things that are going to be useful in the future, long term, while you keep tabs open while they are in current use such as when researching a topic. For example, when someting here breaks or I want to learn more about a given topic, it is not uncommon for me to use hundreds of tabs open while I triage what is useful and what is not. Some of those will be added to the bookmarks (or pocket, or pinboard, depending on how I like them).

My personal workflow usually involves opening huge number of tabs researching something, then closing most of them later. So we can say that there are peaks of tab usage followed by a phase of organizing stuff. The advances in the recent Firefox releases really help me.

But this is my own personal way of working, others may have different views.