[dboreham]

You do it if you're using a language where there's a rule that there are only functions. For example there's no concept of doing one thing followed by another. The only way you can arrange that is to have two functions and compose them: g(f(x)). So now you know that the code in g is executed after the code in f. Monadic composition is basically about doing that while having code that looks like it would in a regular language that has statements and sequential execution. There are a few other reasons, such as you can create your own control flow as-code, lifting, etc. But the primary driver is: because you can't write useful programs in a pure functional language without this trick.