High reputation sites: travel, hotels, the popular marketplaces.
Junk: drugs, mattresses, pills to grow your hair back, cooking recipes, SEO.
1. Everyone knows who the competitors are 2. You are either going to be paying for those services or not, and if you are you will decide which competitor you want to pay for based on something different than how they show up in your google.
Examples - Thomson Reuters' legal services, LexisNexis etc. I'm pretty sure the amount of times some lawyer searches for a term, ends up on some competing legal service and just says oh well I better just pay for this now is negligible.
Near or actual monopolies.
SEO in these cases might help rank, but you don't actually need to rank for your business.
In my experience, it's brand new or still relatively young companies that SEO is often not great for. It might be a great thing to engage in in the future for them, but early stage companies strapped for resources need to be more focused on building out an actual brand rather than on ranking page 1 against competitors who may be spending 10-100x on marketing than them and have been around for years.
If you're an ecommerce shop optimizing product and category pages - SEO will help.
If you're a plumber and ranking higher than your competition will bring in new leads & jobs - SEO will help.
If you're an established SaaS company launching a new product - SEO will help.
If you're a brand new SaaS startup - SEO is probably not the best thing to spend money on. That money is likely better spent on PR getting press mentions, and on PPC ads promoting your brand.
The main problem I see many small businesses making is relying solely on SEO to bring them leads while having no other marketing strategy. This may have worked well for them back when you could spam websites to the top of SERPs, but it's not the case today.