Last year, Google started making developer previews of the upcoming version of Android available shortly after their Google I/O keynote. They've done it with Android L (later named Lollipop) and now with Android M. This is a great way for developers to flash the developer preview on their phone - hopefully not their daily driver or workhorse - and test out their apps with the new version. That way they can be sure that their apps are compatible when this new version of Android is available to everyone.
The only downside is that the developer previews are only available on a few Nexus devices. And not every developer has a Nexus device. Sony recently made the Android M preview available for a number of their Xperia devices. This is good for developers as it allows them to work with a device that the developer preview was not built to run on, as well as being able to work with Sony's proprietary software.
One of Android's strengths is the many, many sets of hardware it runs on. We have the Galaxy S6 which has an Exynos processor, then there's the HTC One M9 with a Qualcomm Snapdragon processor, and even the One M9+ with MediaTek's latest and greatest inside. Being able to use the developer preview on other hardware is not only beneficial to developers but also to Google and the OEMs. As it will help them find bugs before the update is released to everyone.
However, with that being a strength, it's also a weakness. If you look at Google's biggest competitor in the mobile space, Apple. They are able to release betas for all the devices that will get the new version of iOS - not to mention update every device at the same time. This is because Apple made all of the hardware combinations, thus have them all there to test with. Android OS updates will never be as seamless as Apple's, and that's just the nature of the Android platform. Seeing as anyone can take Android and use it.
So while it was great to see Sony take the step to make Android M available for a slew of their devices, it doesn't really help out developers all that much, as it's the exact same developer preview as what the Nexus 6 was running. As there are no Xperia UI elements included, it doesn't really help developers developing for Sony's smartphones either, unfortunately. However, being able to run the developer preview on any phone, and not needing to get a Nexus, is still a big deal, and something that should happen.