Picture three layers:
Promotion
Product
Proof
The first thing that matters is proving there's a need or desire for your thing.
"I know dentists get frustrated with process x."
"I know remote work is making people change y."
"I know I would've built my business differently if I knew z."
For example, you could validate the demand for a software product by building a spreadsheet and running it manually.
No need to build a huge amount of infrastructure before you know whether or not anyone cares enough to spend $.
When real proof is the catalyst for maturing the product, you eliminate a huge amount of risk.
For some reason 90% of people approach this backwards, throwing huge amounts of capital and energy behind things they don't know that anyone wants.
If you have proof, you have a better product, and promotion is exponentially easier.
Satisfied customers, specific insights into the nuanced reasons people choose you over another product or service, all fuel for marketing that will resonate.
Prove it, iterate on it, promote it. Repeat.