What is the benefit of macOS dropping support for 32-bit software?
Apple's explanation for the transition can be found here: 32-bit app compatibility with macOS High Sierra 10.13.4


  All modern Macs include powerful 64-bit processors, and macOS runs advanced 64-bit apps, which can access dramatically more memory and enable faster system performance. The technologies that define today's Mac experience—such as Metal graphics acceleration—work only with 64-bit apps. To ensure that the apps you purchase are as advanced as the Mac you run them on, all future Mac software will eventually be required to be 64-bit.