I wasn't there when it started but wasn't the raison d'etre was the GCC fork that eventually became the new upstream for GCC. Because the fork was faster Gentoo got the reputation that compiling from source was to get faster executable. This is a misunderstanding imho, as the greatest strength of Gentoo the level of customization enabled by use flags.
I would not call egcs that fast, nor do I remember it having any role with Gentoo.
Gentoo was the first distro to use egcs as part of their default toolchain. Not sure if it was the only, but it was the only one I heard to had done so.
I can tell that in 2004, it was a regular GCC, and I got fed up compiling it.
It also made it nice base for both ChromeOS and its descendants like COS and Flatpak (continuation of original CoreOS)