The discipline has three load-bearing parts. Technical work makes pages reachable, indexable and fast. Content work matches pages to the intent behind real queries better than the pages currently ranking. Authority work — links, brand mentions, reviews — gives a search engine a reason to trust one source over another. Neglecting any one of the three caps what the other two can achieve, which is why competent programmes budget for all three rather than specialising in whichever the provider happens to sell.
SEO is slow and cumulative, and that shapes how it should be bought and judged. Results lag the work by a quarter or two, the asset keeps producing after the invoices stop, and the cost per acquisition falls over time rather than rising with volume. Judged month by month it always loses to paid advertising; judged over two years in a market you intend to stay in, it is usually the cheapest acquisition channel a business owns.