I started using Delicious a few years ago after I had become fed up trying to organise my website bookmarks. I often want to bookmark A&D news stories, blog articles, technical papers and standards, etc., so that I can easily find them again when I want to read through them at a more convenient time or when I need to do research for a Wind River article or presentation.
The problem with web browser-based bookmarks is that this approach doesn't scale well for a large number of bookmarks. Web browser bookmarks can be sorted into hierarchical folders, but this can be counter-intuitive if you have inter-related subjects (e.g. safety and security). Also, if you also use more than one web browser as I do, then keeping bookmarks up to date in mulitple places becomes tedious. (I alternate between Firefox and IE depending on which I think is the least insecure at any particular time, Firefox is my current choice due to the NoScript plugin).
Social bookmarking overcomes these issues by allowing you to associate multiple tags with individual bookmarks, e.g. cyber, infosec, safety-critical. This make it much easier to order subjects and inter-related subjects without having to store them hierarchically (e.g. cyber+security+UK). The cloud computing approach also means that instead of tying my bookmarks to one web browser on one computer, I can access them from any computer. (This short video clip from Common Craft explains it much better I than I can). I've found the Delicious bookmarks to be so useful that I even embed them on my personal website so that other people can access them easily.
Sadly, one job I'll need to do over Christmas is to migrate my Delicious bookmarks to another social bookmarking service.