Well, first of all, clay or terracotta pots are porous and air to penetrate it and to reach the roots situated at the edges. These pots also have thick walls that protect the rots, which is an advantage when they are used outdoors. Clay pots also form a white crusty layer on the outside when mineral salt from water permeates the walls. Most consider that an unattractive detail and try to hide it. The following ideas are useful for cases like these but also if you simply want to make your terracotta pots look more attractive for purely aesthetic reasons.