I think it's just a general speedup because there isn't as much bloat. Since everything is loaded into memory, and most likely arrays when stuff like temp, damage, combat stuff Needs to be calculated it has a huge list of SKIN, BONE, MUSCLE, FAT, LEATHER, THREAD, etc to go through to get the numbers it needs to do the math.
But if you cut that down it makes the game able to function at a faster pace. Not only that but the smaller lists help too (why be able to trade/order/see in stocks 500+ types of something, that is functionally all the same? barring stuff like IMP and DRAGON stuff since they are *fireproof*)
Even if one leather is the same as the rest (mostly) it's more for fluff like, hey I'm wearing kitten socks, instead of I'm wearing soft leather socks.
People like myself don't care about what i'm wearing (the fine details, just what type I am wearing and what does it do, stats) I know for creatures I've created a "
General Material Framework" (I have a really ugly HTML doc for what files are what, what CVs do what, and a quick copy. If used right you can very easy use a COPY and REPLACE sections of creatures with the desired CV that'd give them the generic materials) which is really just a series of Creature Variations that more or less manually define tissues but use the material from another creature
EDIT:
Some basic files are added with the GMF for things like PUS, and BLOOD which there are CVs that will make use of those. But if you wanted you can easily just reference what I have in the GMF and learn from and make your own version as you see that you need. Or just use what you think you need from it as you see fit