Maybe relational databases were a necessary step in evolution but they aren't easy for many people to understand. Many common objects map poorly onto relational models, making the convoluted tables difficult. In contrast, XML maps hierarchies and links. Hierarchies appear to be a fundamental way of human thought, manifested in countless classifications and taxonomies. Simply by writing an XML file, you are structuring information.
Links are indefinitely flexible and can represent any data structure.
HTML dealt with human2human communication through text and graphics and it is superb for these processes. XML came out of SGML and SGML was primarily aimed at text-processing. Books like 'XML in 7 days' are meaningless in geek philosophy. Understanding will grow slowly but steadily. Here's how to start.