A great many SEO companies purchase links for their clients. Because of the nature of link buying, in that Google thinks very poorly of it, the SEO companies that do it tend to keep it quiet.
However do it they still do. The reason they buy links for their clients is because, by and large, link buying still works. You can spend a few thousand pounds on links for your website, and assuming you’ve gone to a half decent link supplier, you’ll begin to see results with your search engine rankings.
However, this is a very short-term strategy. The reason for this is that most links you buy aren’t actually bought; they’re rented. You only pay for the link for the term of your contract with the SEO company. Once your contract has expired, the chances are that your link will be removed. So spending all of that money on links is something you’ll need to keep doing, just to remain where you are.
When you add content to your site however, it is permanent. Every page you add increases the value of your site. So if you have a supplier giving you unique content you don’t need to worry about losing it all if you decided to leave them, it’s yours. It’s on your website.
Then if you choose to add more each year, you’re adding to what you already have as opposed to paying to sustain what you’ve got, as in the case with links.
But the biggest reason you should be adding content instead of buying links is simply that content is what Google wants to see, whereas paid links are something Google has openly said it doesn’t want. If you add content to your website you’re doing something that will make Google happy, if you buy links you’re doing something that could incur penalties further down the line.