If you’re a developer creating binary packages, like an RPM, DEB, Flatpak, or Snap, you have to compile code for a variety of different target platforms. Typical targets include 32-bit and 64-bit x86 and ARM. You could do your builds on different physical or virtual machines, but that means maintaining several systems. Instead, you can use the GNU Compiler Collection (GCC) to cross-compile, producing binaries for several different architectures from a single build machine.
