1. Annual recommendations (Reserved Instances, Savings Plans) Sure, if a company only wants these recommendations - they can bring on a contractor once per year to do these changes. This is the one-off fee type contractor the poster is talking about
2. Continuous recommendations (Rightsizing, Spot Management) These recommendations are typically not something you could do once and be done. Rightsizing (which includes zombie instance detection) can happen through time. Spot Fleet management is also a continuous play - spot prices fluctuate and Usage's strategy works hard to maximize savings while minimizing downtime.
You might ask, how does this compare to hiring a contractor full time?
If you're a midsized company, Usage charges $5k/month (or $60k/year). A fulltime contractor in SV would likely be at least $100k/year and that doesn't factor in the number of billable hours (through interviewing) it would take to find someone who knows 50%+ of all of AWS' features.