Whether or not you personally use it often, the stock Calculator app in Android needs love just the like the rest of the stock Android apps, and Google saw fit to revamp the Calculator application within the Android M preview. They haven't changed much aesthetically, which isn't always needed in the first place, but even without any major changes to the UI design Google seems to have still given the calculator app some adjustments and tuning, along with some new features which may be more useful to anyone who uses the app then to those that barely touch it.
There is also supposed to be a decent amount of bug fixes in this particular version of the calculator which should be pretty commonplace by now, as most app updates or new versions tend to include performance and bug fixes. When it comes to new features, Google has added in the ability to copy and paste text to and from the calculator. This might not be something you use often, but it is extremely useful for the times you do. Imagine wanting to know the price of something you see online when multiplied by a specific number. With Android M users will simply be able to copy the text of the number figure into the calculator and add the appropriate remaining calculations to get their answer. Nifty.
In addition to the new copy and paste function being present, Google has added in modes for anyone who needs or wishes to have trigonometry tools at their fingertips. Users will be able to change modes to RAD or EDG within Android M. Most people might not have a need for that in their day to day, but there will be those that do and it's nice to see Google accommodating. Also in the M preview version of the app, floating point arithmetic is no longer used and Google has removed it. This is largely due to accuracy issues with that particular method of arithmetic, so if you were having problems with accurate calculations before, or have been experiencing other bugs, chances are the M preview cleans things up on this front.