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Why do people use extendable algebraic patterns when polynomial -monads are conceptually simpler and have the more clearly interesting generalization of -monads in general? Is it similar to the reason Lurie defined -operads as -categories equipped with a certain kind of functor to pointed sets, i.e., it's just easier to compute with?
I suspect a combination of easier to compute with (an algebraic pattern is usually a fairly small and concrete category with a useful factorization system) and easier to construct. It's also sometimes helpful that symmetric monoidal categories are a particular algebraic pattern (in much the same way the fact that this is useful for $\infty$-operads). Also, I tend to think of algebraic patterns as a bit simpler than polynomial monads, so it's a matter of perspective and taste as well :)
Also, nice algebraic patterns are more structured than an arbitrary polynomial monads (I believe that they correspond to complete ones).