According to Google, “In order to improve stability, performance, and security, users who are currently on 32-bit version of Chrome, and 64-bit Windows with 4GB or more of memory and auto-update enabled will be automatically migrated to 64-bit Chrome during this update. 32-bit Chrome will still be available via the Chrome download page.”
Basically stability, performance, and security are the main reasons why you should upgrade to the 64-bit version or should be using it anyway, and moving forwards Google will be ensuring that will happen for its users. However this doesn’t mean that the 32-bit version will be going away as Google still plans to keep it around, just that users on compatible computers will be automatically upgraded instead.
Note that this only affects Chrome 58 so if you’re running an earlier version of the browser it might not affect you, unless you have already updated or have set updates to automatic.