If joint is designed visible it can be used as decorative element in the design. Joints like slot joint, and dovetail joint can be decorative elements of the product.
If joint is designed invisible then decorativeness is of minor importance. The design can be affected where and to which direction the seam of two joined components is situated. Also the surface characteristics of wood affect a lot to the look of the product. If the seam is designed as invisible as possible then the demands of manufacturing quality are very high. It is often better to design the seam visible with little chambers at the edges. Then it is not so easy to see little cutting inaccuracy.
The dimensions of the joint have remarkable affect to the appearance of the joint. If the joint is designed with a lot of strength it may look clumsy. If other structures are designed slim as with form pressed components, it may be hard or impossible to make the joint strong enough.
Designer may want to use fixed, invisible joints but then transport may be too expensive and one has to choose open able joint.