When releasing a product for free, just about every developer simply assumes that consumers will be willing to pay for it in some way, they assume that there will be a reliable revenue stream in the next year, or two years. Conversely, quite often there isn’t, and startups go bust. If a product is released on a pay-only basis, you know in the first month if the product will succeed, rather than two years and thousands of dollars down the road.
It's not quite so clear-cut, but this article makes a good point that you should think carefully about whether to have a freemium model, or a pay-only model. In the past few years, many startups fell into the freemium model by default. Some have, in time, realised that this was not right for their products. Others have sunk.
Some products (for example, many enterprise products) are ill-suited to freemium. Think (and test) carefully before choosing freemium for your business. Above all, avoid choosing freemium implicitly, without even making a conscious choice.