Now that multilib has a definition, it is time to decide whether to enable that support. Depending on what you want out of this book, it may be required.
Steam has a hard requirement on multilib capabilities and demands 32-bit libraries on x86 systems, whilst at the same time not supporting pure 32-bit systems. In order to run Steam on Linux, you need to have an x86_64/AMD64 CPU and many 32-bit libraries, including Xorg, audio, and GPU drivers.
Originally, Steam was a set of 32-bit binaries and began introducing 64-bit only components, like for the store front and eventually the log-in screen. Many have asked for years that Steam should become fully 64-bit. The goal was to no longer require 32-bit libraries at all, no multilib needed. Steam eventually only fulfilled one part of that, which was making Steam 64-bit; at the same time, Steam still requires 32-bit libraries. This will be the case for the foreseeable future. Multilib is absolutely required for Steam.
Wine can depend on certain things, differing upon the architecture being built. As these architectures require a more in-depth explanation and a roadmap on how to approach things, see Wine Dependencies which will help you better understand whether you need 32-bit libraries. If you need a given architecture and it says it needs 32-bit libraries, lib32, or MLFS, proceed to the next page. If you don't need those libraries but need the i686 MinGW-w64 toolchain, you won't need multilib for Wine. As a teaser though, if you want the new WoW64 or pure 64-bit mode, multilib is not required for Wine.