Exposed joinery may be justified in one or a couple ways:
A particular joint may be easier to make than its unexposed alternative. A wedged through mortise and tenon demands less skill than a fox-wedged blind M&T to fabricate the same joint. Likewise, exposed dovetails are much easier to make than those hidden inside a miter to join panels at their angled edges.
Exposed joinery may also be integral to the architecture of a style of furniture, such as Gothic and Arts & Crafts, nominally allowing a piece to be rock steady in use but visually disassembled.
Sometimes construction, the maker's skill, and esthetics combine to rationalize the use and inclusion of emphasized exposed joinery to put unique and different materials together in a pleasing way. Heres a console table that started with a flitch of Chinese Chestnut that I wanted to present as flamboyantly as its grain and shape suggested, by including contrasting wood, etched glass, leg profiles, and proud exposed dovetails in a mutually complimentary way.