Google Plus Photos has been around for ages now, it's been a boon to people who take a lot of cameraphone shots for a few years. I remember it's release, getting myself on there as soon as I could. I'd picked up an HTC One M7, and was happy that I could backup full size photos to the service, even though they weren't much bigger than the free tier of backup at 2000x2000px.
Google Plus Photos was released alongside Google Plus, which was the more popular service at the time, but times change. Google Plus is thriving, despite what reports may claim, especially in Asia despite the lack of the Chinese population. It might not have grown exponentially as Google may have wished for, but it is still growing, slowly. What is being used a lot more, though, is Google Plus Photos.
Not everyone wants to be part of Google Plus. They've got their own networks already that they're happy with, be that Facebook or Twitter or whatever. Google knows this. What the vast majority of uses would like though is a good backup solution for their photos. When you've wanted to use Google Plus Photos in the past you've needed the social network aspect, and some people just find it confusing to get into. I've put people onto G+ Photos before, and they've given up before getting through signup, because they don't want Google Plus.
Those who have got into Google Plus have oftn found it confusing and frustrating to find their way around. There's just too much in there, and whilst it might be a lot simpler than Facebook, they've been on Facebook for years and its evolved with them. Breaking photos out of Google Plus simplifies the interface by another step, making it that but easier for newbies to get started.
That advantage follows into the new Photos service as well. Photos gets a lot simpler to use. You don't need Google Plus anymore, which takes away the need to navigate around Plus to get to photos (especially on the desktop). Photos can also add new features without confusing users. This includes a lot of new animation options and enhancements. It means Google can add a lot more into photos without worrying about Google Plus.
This makes Photos a really useful service for everyone, whether you want Google Plus or not. Who doesn't want a simple backup solution for their photos that works without them having to do anything other than enter their gmail address? It makes Photos a lot less confusing and easier to use, and to some extent does the same for Google Plus as well. It certainly doesn't spell the end of Google Plus, just the start of a much better standalone photo service. It makes a lot of sense to split the two up and let the focus on what they're supposed to do, rather than cramming everything into one service that not everyone wants.
One last worry to address was the fear that this might be making it more difficult to share images between the two services, or that some functionality might be lost. I'm happy to say that's not the case and hopefully we'll still be enjoying making ourselves into zombies and the like fit a while to come.