Search engines used to sell rankings (positions in search results) as part of their business model. To get ranked in search engines in the early days didn't require a lot of search engine optimization (SEO) skills or effort, but just that you had enough money to get put at the top of the results for your chosen keywords or phrases.
Then along came Google.
Google made a name for itself in the early days, competing against corporate giants with huge backing like Yahoo, AltaVista, Inktomi, etc. It got that name by eschewing the 'profit motive' of the other search engines and becoming the first successful 'editorially neutral' search engine option. In other words, instead of selling itself to the highest bidder or taking on a political or social slant as many of today's 'news outlets' have done, Google remained neutral and without bias. The Switzerland of search.
The only ways a company could rank well in Google and be seen were to pay for advertising in the clearly marked Google ad spaces around search results or work hard to make themselves relevant enough through SEO to be the top contender in Google's fickle search algorithm.
In standard search results and even in most specific searches, such as Google Images or Google News, paid inclusion is still not present, but in new launches like Google Hotel Finder and Google Flight Search, both launched last year, and in Google Advisor (their financial services search), are including paid search inclusions.
This became clear when Google launched the 'comparison boxes' for looking at results side-by-side in a product comparison-style mode. Some results in those comparisons, even though these are supposedly editorial-free side-by-sides, will have 'sponsored' labeling them as paid for. This is because Google had to get the data by paying for it, often through 'trade' relationships with data providers which result in the 'sponsored' label.
To its credit, Google is making it pretty clear that those who're paying to be there are labeled as sponsors rather than left to appear as organic matches. That's a good sign, but it opens the question regarding whether paid inclusion could begin migrating to other search results.
After all, the company has little competition and virtually controls the search market. On the other hand, the three competitors listed above are all no longer relevant in search while Google dominates. Were the company to move to a less transparent model and include paid inclusions, they would likely quickly find themselves losing share to an upstart competitor who 'pulls a Google' and gains a reputation as being editorial-free.
Whatever happens, it's important to watch. The paid inclusion model has largely failed for most search and resurrecting it would likely end in disaster for mainstream search options.