Comment by Freak_NL

Even on GNU/Linux distribution it is a settings that must be enabled first, but at least it tends to be present as a first-class option right from the start.

I can't stand those weird alt-codes Windows users end up memorizing. You only know those characters for which you've learned the codes. With the compose-key, you can guess anything that is diacritic character, and lots more besides¹.

I showed a colleague on a Windows computer WinCompose when she got wanted to be able to easily type a number of special characters (like en- and em-dash) without having to hunt for them in a character map or remember arcane alt-codes. She's not a developer, but it clicked instantly. Just think of a logical sequence, and get the character you want (for en- and em-dash its [compose - - .] and [compose - - -], respectively).

1: Like superscript numbers. Once you know that [compose ^ 1] yields ¹, you know ²³⁴⁵⁶⁷⁸⁹⁰ too.