It's actually better this way I think. Lots of people, even if they are only marginally interesting in Fate, likely watched Zero, and we're always wondering why Sakura looked so happy now when they watched UBW. And even for complete newbies, this manga would still be good because it gives concrete info.
The VN is great and has lots of lore, but it also has a lot of parts in HF that are left vague or open-ended, and the movies are likely to skim and trim a lot of actually crucial to better understand the characters (I hope not, but there you go). So this medium might be the best for them to understand things on a whole without spending a huge amount of time in Nasu's wordswordswords, but still have much more information and context than the movie.
Since iirc, it was said Caster was taken out by a ploy by Assassin's master, and we know who that is. So the manga will gives us a sort of interlude that shows how Zouken broke Caster with deception to the point she hardly noticed Saber and Shirou storming the temple. It'll surely make the part where Zouken reanimates Caster that much more saddening.
Man, the UBW anime showed that she at the very least cared enough to free those orphans her former master made as sacrifices, it really makes me wonder if they couldn't really talk things out if everyone knew how much bull the grail and the war really were. Really makes me sad.