Even as we know that SEO is not all about the links, we acknowledge that it is a big deal. It used to be a great thing to have a website linked to yours, through something called reciprocal links. Well, search engines have caught on and have now rendered those reciprocal links practically useless in getting higher ranking. For search engines, mutual links seem too fishy and most likely meant there's some sort of connivance.
Search engines have grown so smart when it comes to link analysis that they could figure out how pages or websites are linked amongst each other. Even more reason to remember that the value of link building does not lie on the quantity but on the link's reputation and topic. Why so? Well, a website about the same field as yours will count more than those that are not related at all. For example, you have a website about a restaurant, so it will make more sense to link to food blogs and directories and whatnots instead of linking to DIY construction projects, right?
Bookmarking is an important part of quality link building. It's the perfect way to save the links without having to store the bulky webpages themselves to access it easily next time (kinda like how you would bookmark pages of a physical book to mark where you left off). Online (or offline) bookmarking tools enables you to edit descriptions, like filenames, or add tags to make them easy to search (a strong factor for this to be added in SEO strategies). You can also share it to others and add comments or ratings to other's bookmarks.
However, there's no standard way or structure in putting tags to bookmarks which is double-edged: you can use any keyword or phrases you want but then it's hard to access other bookmarks that are tagged unconventionally. All the same, in the eyes of an optimizer, a webpage submitted and tagged multiple times increases its chance of being found by search engine (and provides you with those precious backlinks).
And last time we checked, forum sigs still count, as do blog commenting. (Yeah, we know stuff like that.)