In fact, the 32-bit version of Outlook is even offered by Microsoft Outlook 365 by default on new installs and is much more common than 64-bit Outlook.So to be sure, it is necessary to find out whether you have Outlook 2010, 2013 or 2016. In that column look for “Help“…if you see it, then you have Outlook 2010 and you can see whether you have 32-bit or 64-bit Outlook after the version number: If you don’t see “Help” but you do see “Office Account“, then click that.
The main reason for the original recommendation was compatibility with existing add-ins for Office.
I seem to recall that Microsoft used to recommend to install the 32-bit version, even on a 64-bit version of Windows but that was some years ago.
Microsoft indeed still recommends installing the 32-bit version of Office unless you have a specific need which requires the 64-bit version.
For an add-in to work with the 64-bit version of Office, it needs to be recompiled by the developer specifically for the 64-bit version of Office.
In some cases the developer also needs to make some specific changes for 64-bit support or needs to wait for specific libraries, that the add-in relies on, to be recompiled for 64-bit.