For the most part the answer for this question is YES because the device diminutions are pretty much the same and all the ports, camera and buttons haven't moved so chances are that you are probably be able to fit your iPhone 6 old cases on your new device.
I believe that these are great news because many people still have a useful case in a great condition that can be used for the new iPhone and you don't have to spend more money on a case if you already have a suitable one. There are some minor changes in the phone size though, but if you are a hard case it will most probably fit without any issue.
If you are about to try and fit your old iPhone case on an iPhone 6s it might be a completely different story and the case will simply not fit the device because the 6s is bigger and wider than your iPhone 6.
In general cases are always a very important party of your daily iPhone use and it is always a good idea ti have a solid protective case covering your iPhone. If you still don't have a proper case you should know all the great benefits that can come with a case. Style, better grip, overall increased protection and much more are some very nice reasons why most people have to use smartphone cases in order to make sure that their device survives for a longer period of time.
If you are looking for some great iPhone 6s Cases you can try these iPhone 6 cases which will fit your device without any doubt:
Best Cases For Samsung Galaxy 6s/6
Not so long a ago smartphone cases used to be bulky and less effective in protecting your smartphone than they are now but today, even though they come in different shapes, color and toughness level, most of the smartphone cases are extremely protective and much easier to install.
There are many different styles and use cases that a modern protective cover can fulfill. Personally I really like wallet cases because the both protective and very useful on a daily basis.