Apple hasn't supported 32-bit Macs since 2011 when Mac OS X 10.7 Lion was released. Obviously, it's been quite a while since Apple made a 32-bit Mac at all.
You might even think that Office 2016 is a 64-bit application because of this. It's not. Today, Microsoft is finally making a 64-bit build of the productivity suite available, but only to Office Insiders for the time being.
If you plan to install it, Microsoft has some notes for you:
What does this change mean for me?
If you use Office 2016 for Mac but don’t use any add-ins, you should be unaffected by this change.
If you use or develop add-ins in Office 2016 for Mac, you may need to update those add-ins to 64-bit.
Please note: If you use the Thomson Reuters EndNote Cite While You Write (CWYW) add-in for Word 2016 for Mac, and you need a 64-bit version of the add-in, consider joining the EndNote beta testing program. Once you sign up, you’ll receive a beta version of the 64-bit CWYW add-in. Any feedback you provide will help ensure that the add-in for EndNote is fully compatible with the 64-bit version of Word 2016 for Mac.
I develop add-ins or write VBA for Office 2016 for Mac. What do I need to know?
The work required to update your add-ins for 64-bit will vary based on the code.
For example, any use of VBA’s DECLARE statement to load external functions in compiled dynamic libraries will require that those libraries support 64-bit. The VBA code itself will continue to work as VBA is transitioned to 64-bit, but you’ll need to update the external libraries.
If you're not familiar with why Microsoft has not yet made a 64-bit version of Office for Mac, these notes should shed some light on it. Certain 32-bit applications and add-ins won't work with 64-bit Office. In fact, if you install Office on Windows, 32-bit still shows as Recommended.
Of course, add-ins are new to the Mac to begin with. This was a feature that was announced at Microsoft's Build 2016 developer conference.
So why do it now? It's simple; developers are asking for it. In fact, one user said that he has a Windows partition on his Mac just so he can boot it up and use 64-bit Office. Developers want larger address spaces.
If you want to try out 64-bit Office for Mac, you need to be an Office Insider. If you already are, you'll have the option to update to 64-bit; if not, you'll need enroll through AutoUpdate, which you can find by going to Help -> Check for Updates.
Source: Office Dev Center