Reviews are public accounts of working with a business. They affect two separate things: whether a prospective customer chooses it, and where it appears in local search results.
What is read
The average rating is only part of it.
Volume — a handful of reviews carries little weight regardless of the score.
Recency — reviews from years ago are discounted by readers and by search ranking.
The response — a considered reply to criticism is frequently more persuasive than the complaint is damaging, because it is evidence of how problems are handled.
A rating that is perfect and small is generally trusted less than a strong rating across many.
Getting them
The reliable method is asking every customer at the point the work is finished and they are satisfied, rather than occasionally and selectively.
Timing dominates. An hour after completion produces far more responses than a week later.
Incentives are restricted by most platforms and by consumer protection rules, and a review that is bought is a liability rather than an asset.
Where they show
Reviews on the business profile that serves local search have the most effect on being found — see Google Business Profile.
Against paid marketing
Reviews are earned rather than bought and do not stop working when spending stops, which makes their effective cost per lead fall over time while advertising does not.