When I started playing in the early 90s, the table was I think a hangover from an earlier more RPG-ish iteration of the rules. There were other things a GM might think of using the WS stat for in the earliest iteration, it wasn't necessarily just for determining whether you hit your c/c opponent. So while granular differences of WS might not be significant when it came to hitting your opponent, the stat differences might be significant elsewhere.
On a note of pure pedantry, I'm not sure that's
quite right - the first-edition rules did nominally include an RPG, but the WS stat was just for fighting. The RPG is very barebones, and there's no guidance at all on using the stats creatively - or even for standard RPG activities such as climbing or sneaking. The RPG is just the combat and magic system plus character generation (which includes skills without any resolution mechanism).
And the scenario provided (The Redwake River Valley) is a more of a series of linked tabletop skirmishes than a proper RPG adventure: there's a lot of player decision-making and the opportunity for some limited roleplaying, but it's really just to shape how the later battles will play out - in many ways, it's a forerunner of the classic scenarios for Warhammer 2nd edition, with opportunities to recruit troops for the key encounters or to lose them in clashes with wandering bands of monsters. It's more Orc's Drift than Keep on the Borderlands. That's not a bad thing - it looks like it would be great fun to play out.
I think it's more the case that the wargames of the 60s and 70s were very big on tables generally - and that Warhammer was put together quickly and not heavily playtested (Rick Priestley says as much, if memory serves, in a really interesting recent YouTube interview). The table is largely unchanged since the first edition (hitting has become one pip easier, and they now bother to provide target numbers for WS over six, but that's about it). For the most part, WS becomes largely redundant after a certain level - in the first edition, there's no difference between having WS 8 or WS 9.
Where the RPG side did play a part, I think, is in having WS go up to 10 (so characters could keep advancing for a long while and eventually be able to fight balrogs, etc., one on one ). In the first edition, WS 10 is
marginally better than WS 9 - but the only advantage of being WS 9 is that you're closer to being WS 10, which means you only need a 2 to hit a foe with WS 5 rather than a 3 as at WS 8 or 9. (I'm pretty sure no one ever played the RPG for that long, though I'd love here about it if people did ...)
(After looking through The Redwake River Valley, I'm quite tempted to give it a go with the kids over the Christmas holidays - I just need to be sure I have 2d4 dwarves and 3d4 sea elves to hand! Maybe the Ziggurat of Doom first, though we've played that many times using Song of Blades and Heroes.)