- they keep sessions (a)live (audio/video, chat, where you stop reading)
- they can keep some or all of the history context ("how did I get here?")
- they can keep some of the browsing context ("what else was I looking at, what other tabs are around this one?")
- they are a filter. Every useless site, popup, get's into the history (>"If they don't close it, they want to remember it forever.")
- they (at least on desktop) have better ux than bookmarks and history (they're always there)