From the title of this thread, it seems to me that the question is: "Why purchase sceneries when there are so many available for free?"
From my experience, the commercial products are usually very accurate and include features like animations, textures for all seasons, night lighting, etc., while the quality of freeware varies and is completely dependant on the skills and tools of a single creator. Some freeware is very good but on the other end of the spectrum I've downloaded freeware sceneries for Montana where the creator, armed with a cheap design program and some free object libraries, had simply reimagined international airports as RV parks surrounded by log cabins...simply because that was their personal but uninformed perception of what Montana is like.
If the question is, "Why use airport sceneries at all," the answer is for enhanced immersion. Flight simulations are popular for a wide variety of reasons, from training and keeping up proficiency to outright fulfillment of fantasies. Adding custom sceneries match the virtual world to our personal reality; they allow us to explore places we may never actually visit; and they even let us experience
bygone eras .