for some, reading the manual was also fun. For most, reading the manual was an admission of failure.
I think I was the only one who really cared about it. I've had it for months, I think. Read it cover to cover more times than I could count. Pretty much memorized every bit of lore and trivia there was in it - and there was quite a lot. The manual itself was a small booklet, with a couple pages of backstory for each of the playable species, plus a lot of extra world-building details about individual units, buildings and planets. So even as I didn't have access to the game, my own imagination built a whole world and infinite stories out of what some would consider "filler content". But this filler content made me care, and eventually shell out my pocket money for Brood War, and later on, money from my first full-time job on the collector's edition of StarCraft II.
That SC manual came probably at the tail end of the era where the publishers put any kind of effort into manuals. Case in point: SC:BW came with just a tiny booklet with about equal split between lore/context, instructions for the new and changed game aspects, and legal/credits. SC II came... well, I don't know with, by that time Blizzard stopped caring about the game story anyway.
(Oh, and speaking of warm memories of computer manuals in the 90s, I have a special place in my heart for the big, heavy, couple hundred pages long manual to Word 6.0 (not 100% sure about the exact version), and earlier, for the interactive self-training application that came included with Windows 3.11 - that one taught me how to use the mouse!)
More interesting in retrospect, the first manual contained extensive descriptions of kiith clans. They go on for dozens of pages, and don't show up in the game's actual story at all. Someone spent serious effort writing them. Why? Left over from an earlier version of the game pitch where the player would have to manage relations between competing groups on the mothership? https://sierrachest.com/gfx/games/Homeworld/box/manual.pdf
Skimming from the dozens of pages I would love to have read back then, I think this may be the case of writers creating a detailed setting for the game early on - only fraction of it could become part of the gameplay, but presumably designers and writers were using it as a reference. And, come release time, it didn't hurt to include the world-building bits - it might just get some players to enjoy the game more (I know it would've had this effect on me), while the alternative, back then, was for those details to never see the light of day.
And/or maybe they hoped to make some follow-up games, or tie-in novels, etc. - then including those details was both important to solidify the fictional universe, and would drive some secondary sales for the game itself.
Pretty interesting idea that eschewed technological tricks to prevent copying.
Which specific version do you distinctly remember?
Some board games come with pretty hefty manuals, still. Twilight Imperium comes with a whole separate lore booklet that has nothing to do with playing the game.