A lot of languages have a staple in their language as the traditional branching structure, some form of "IF some condition is true THEN perform some statement ELSE do some different statement". Typically, then, if and else become keywords of the language; they are reserved and can only be used in the formation of branching statements.
Programming if condition then else is simply a way to express a fork in the execution path. We are saying, based on some condition, we should either execute one set of instructions or another. But as we saw before, pattern matching is quite expressive to this end. Based on structural or value matching, we can execute one version of a function, or another. What motivation, then, is there for expressions such as if true do x = 42 end? The expressiveness of pattern matching is occasionally too high-level when expressed in terms of functions. We need something that is closer together, but still expressively simple and easy to reason about....