It is a fair question: exchanges publish order books and options chains, and the APIs are free. Why pay anyone for a derived view of public data?
The answer is that the derived view is where all the work is, and most of that work is invisible until it is missing.
Normalisation
Every venue names things differently, quotes in different units, uses different conventions for contract size and expiry, and changes them without much warning. Making one coherent view across several venues is continuous work, and it is the part that breaks quietly when a venue ships a change on a Sunday.
Judgement about what counts
A gamma profile computed over every listed strike including ones with no open interest is noise. Deciding which books are real, what the size threshold is, how to treat a venue with thin liquidity — these are choices, they materially change the output, and they are what separates a useful map from a chart of nothing.
The history
This is the part you cannot build retroactively at any price. A percentile needs a distribution and a base rate needs cases. Both come from having recorded the derived view continuously for years. Starting today means having that in years.
The honesty layer
Freshness on every reading, named errors instead of silent zeroes, withheld fields listed rather than zeroed, a published record of what the guarantees are. This is unglamorous and it is the difference between a feed you can build on and one that will eventually mislead you at the worst moment.
What it is not
It is not access to anything secret. Every input is public. If a vendor implies otherwise, they are selling the wrong story about their own product, and it is worth wondering what else the story is covering for.