Here we go again. Why would someone like Ilford even bother to market two films in the same nominal niche if they were so similar? - say,two approx 100 pan films with fine grain and med curve slope? (FP4 vs Delta 400). Delta has a tiny bit smaller grain; but in moderate contrast conditions I could make almost indistinguishable prints with either of them. But if you read the tech sheet there is the implication that FP4 is marketed as a broad-usage "commercial film". Why? Let just say that for every sheet of FP4 I've shot in the field, I've probably used 50 sheets in the lab. And most of this usage is related to exploiting the characteristics which it does not share in common with Delta. Like what? Color print masks, where the exact characteristics of the toe are paramount. Black and white interpositives. I even know a person who makes color separations for dye transfer and color carbon printing using FP4. While FP4 is not ideal at any of these applications, it is good enough to actually be useful for all of them. That seemingly insignificant difference in Delta 100 would render it a very clumsy choice for any of them. This isn't really an esoteric tool set. Back when the Ilford label marketed both Cibachrome (Ilfochrome) and FP4 as its recommended complement for masking, they probably sold significant piles of FP4 for just this kind of work.
My point is not to convert black and white shooters into color photographers, but to give just a idea of the tip of this iceberg. For many
APUG types, the distinction is really nitpicky. For some of us, it's extremely important. Besides color work, there are many many times
where even films in the nominal 100 category (FP4 vs Delta vs ACROS vs TMX etc) give remarkably different final looks to the print. For
example, TMX has quite a steep toe, so will resolve deep shadows quite crisply. But it has poor edge effect in most developers, so might
need a supplemental unsharp mask just to heighten that effect (which is exactly why the PS faux equivalent is labeled "unsharp mask").
But that edge softness (not lack of detail) often favors portraiture. With Delta, it's difficult to get really deep blacks with gradation in harsh
mtn and desert light. The best films for these were true "straight-line" films with an extremely short toe, like Super-XX, Bergger 200, and
Fomapan 200. But TMY400 works reasonably well, having a reasonably steep toe with the good edge effect TMX doesn't. I could write a book on all this damn stuff and illustrate it with example of my own work. But why? People can simply try for themselves if they're interested. Otherwise, a workshop presentation would be worth a thousand times as much as these endless web squabbles and their relatively worthless web-mauled imagery.