In-world stores have advantages for customers and potential customers:
- An in-world store can show you how the product really looks in world.
- An in-world store can be a meeting place. Anyone you find there is likely to share your interest in the type of item the store sells, and you can trade information and landmarks and even make new friends.
That said, I understand why a creator might choose to sell solely via the Marketplace, and can't hold that choice against any creator. Linden Lab, maybe, but not the creators. I am amazed, especially given the RL economy, that anyone buys land given tier charges.