One of the big new features in Marshmallow is the native fingerprint support. That's part of the reason that the Nexus 5X and Nexus 6P both have fingerprint readers - both of which are blazing fast, I might add. Samsung has had fingerprint readers in their phones for a few years now, but no Android has proper support for fingerprint readers, which is a great step. Now you are able to login to apps with your fingerprint, much easier and faster than using your password. However, something else that's new in Android 6.0 Marshmallow, is the ability to authorize purchases in the Google Play Store with your fingerprint. So now when you're going through the check out process, instead of typing in your password, simply place your finger on the fingerprint reader and you're all set. Of course, this only works on Android 6.0 Marshmallow right now. So really only the Nexus 5X and Nexus 6P can do this.
In the settings for the Google Play Store, you can set your fingerprint as authorization, unless you wish to enter your password in each time. Which you probably don't. As using your fingerprint is much quicker and painless. Of course you can also still use both your fingerprint and your password without changing the options. At the bottom of the prompt, you'll see a green button for "Use Password". So that's nice as well.
Now obviously, this requires Android 6.0 for a few reasons. The big one is the fingerprint API's that are a part of Marshmallow. Those API's are needed in order to use your fingerprint in the Google Play Store. Hopefully more developers are looking at these new API's and will use them in their apps. For example, with Samsung's devices, you can login to LastPass with your fingerprint, which makes things so much nicer when you need to enter your password. We should be seeing more and more apps popping up in the Play Store with support for using your fingerprint as authentication. Which is going to make things much easier for many of us. Fingerprints also make our phone more secure. Which is always good.