In announcing OS X 10.10 Yosemite, Apple did something unspoken: it released a public beta version to everyone who was interested in testing the system. Of course, the company did not release any versions to the general public it left to do so when OS X 10.10 was already relatively "mature" and with acceptable bugs.
But what does she gain by doing this? In theory, users interested in testing a beta system know that everything is still unfinished. The idea here, then, is to help the company by sending error feedbacks at the same time as having early access to something that will only reach the great public later on.
It looks like it worked. According to 9to5Mac, Apple plans to do the same with the iOS. Everything should start with version 8.3 which is already in the hands of developers and brings news such as new Emojis, wireless CarPlay and improvements to the Google account registration screen.
According to the website, Ma plans to do this in mid-March, when the system is even more stable with the idea that the maximum number of bugs be eliminated in the test version so that everything comes perfectly into the hands of end users. The same thing should happen with iOS 9 (it has everything to be presented at WWDC 2015 and win a public beta version a few weeks / months later), being definitely released for everyone between September and November.
It remains to be seen whether, like the users who tested OS X 10.10 Yosemite, those on iOS will be aware that this is really a beta version (of tests).