Why should you care whether an app is 32-bit or 64-bit?
From this Apple developer page:
At WWDC 2017, we announced new apps submitted to the Mac App Store must support 64-bit starting January 2018, and Mac app updates and existing apps must support 64-bit starting June 2018.
32 bits allows you 2-to-the-32nd addresses:
2^32 = 4,294,967,296
That’s 4 gigabytes of addressable space. A 32-bit computer can’t have more than 4 gigs of memory. A 32-bit program can’t directly address more than 4 gigs.
64 bits, on the other hand, gives you access to 2^64 which is equal to 2^32 times 2^32. Clearly, that’s a way bigger number. I won’t say we’ll never need more than 64-bits of addressable space, but I can’t imagine that need in my lifetime.
So how to tell which apps are 32-bit and soon to be end-of-lifed?
Easy. Go to the Apple menu, select About This Mac, then tap the System Report… button. In the page that appears, scroll down to the Software section (in the list on the left) and then tap Applications. Wait a minute or two while the list is built.
Once the list appears, widen the window so you can see the column labeled 64-Bit (Intel). If you tap that label, the table will be sorted into the haves and have nots, 32-bit apps on top, followed by 64-bit apps.
For me, the vast majority of 32-bit apps are legacy holdovers from previous installs that the migration assistant brought along during various system updates.
Why doesn’t Apple let you have both? In a nutshell, supporting both flavors means Apple needs to maintain and ship 32-bit and 64-bit versions of all its supporting frameworks, essentially doubling their workload as well as the size of the OS. In addition, both 32-bit and 64-bit frameworks are loaded into memory, doubling that part of the memory footprint.