When meaningful results are needed here and now
SEO is a long game. Even with regular work to improve the site, it takes a long time to gain the trust of search engines. So you will see high-frequency queries (which users often search for and where the competition is higher) in the top search results in six months or a year. In low-competitive areas this period is shorter. But if you need instant payback or urgently need to sell seasonal goods, it is better to invest money in contextual advertising or integration with bloggers.
Example: a multi-page website for an annual film festival with year-round merch and a blog makes sense to promote in search. But the landing page with the film premiere is not.
When you are not ready to improve the site
With a high degree of probability, in the process of website promotion you will have to modify something - add new blocks, new sections, improve the technical aspect. Over time, the design of the site also becomes outdated; it should be updated periodically. Therefore, a developer almost always works in tandem with an SEO specialist.
Your niche is occupied by aggregators
Competing in search with marketplace giants only makes sense if you have sufficient budgets for multi-channel marketing. It’s better for small businesses to connect to these same marketplaces, earn money there, and only then invest in their own website.
There is no mass demand for your product
If you're selling innovative wind turbine blades, it's better to hire a smart sales person instead of SEO.
Firstly, in narrow niches your site will already be in the top - simply because there are few competitors. Secondly, SEO promotion does not create demand, but only helps customers see you. If your product is simply not searched for, then the search engine will not show it to anyone.