With the 64-bit architecture being pretty much the standard these days for both mobile and computing devices, it makes sense that companies are trying to push users and developers towards running 64-bit apps. With Apple, the update to iOS 11 will see older 32-bit devices no longer supported, and it looks like that will be applied to Mac apps as well.
During Apple’s Platform State of the Union keynote at WWDC, the company informed developers that macOS High Sierra will be the last release of macOS that will support 32-bit apps “without compromises”. This means that starting in January 2018, new apps that are submitted to the Mac App Store must be of the 64-bit variety, and that all existing apps in the App Store will have to be updated to 64-bit by June 2018 if they wish for their app updates to be approved.
For the end-user you probably won’t notice anything, at least on the surface, so this is more for developers. To Apple’s credit, this isn’t a last minute decision that they’re springing onto developers. The company has given iOS developers quite a while and has sent out various warnings and reminders to get them to get their apps updated to 64-bit, a courtesy which Apple has stated that they plan to extend to macOS developers as well.