Purpose can be inner-driven as well as outwardly imposed. For instance, animals mate for the purpose of creating offspring, but there is no evidence that a separate supernatural being either designed the process or inspired each, or any, instance of it. There is purpose, but driven by the organism, not an outside force.
The process of gradual, small, but cumulative developments can produce the appearance of design by eliminating the intermediate steps. To build an arch, you cannot just stack stones over air, because they fall. You need to build a supporting scaffolding until you can put the keystone in last, when the pressure of the stones keeps them in place. Then when you take the scaffolding down, everyone wonders how the arch was made.
A similar process happens organically. Duplicate genes switch functions, code for a physical development, which provides an advantage; the development continues to be modified, and some examples disappear. Eventually, modifications built on other, now vanished, modifications result in an organism where complex parts all work together, and Michael Behe comes along and writes a book claiming that the structures could not have evolved all at once because they wouldn’t work. Creationists start dancing on Darwin’s grave until someone points out how other evolved structures modified their functions to provide enough of a prototype of the supposedly irreducibly complex organism that it could survive and develop into the complete organism we see today. Creationists ignore this and keep dancing, but everyone else laughs at them until the Westminster Abbey security forces make everybody go home.
