While people have made jokes and memes about the body armor used by the Imperial Stormtrooper corps for being "useless", the actual in-universe lore from a lot of sources by both West Endgames (The Imperial Sourcebook, Rebel SpecForce Handbook, etc) along with the Imperial Handbook (which shares lots of the same information the West Endgames Sourcebooks) do, this honestly explains quite a bit and also makes a lot of sense as well.

Real world body armor protects from various things by dispersing and dissipating the energy from lots of weapons historically speaking, so it makes sense as well that in a universe that has master space travel for several millennia, and created advanced energy based weapons that body armor that protects from said weapons also exist logically speaking.

Not to mention the fact that it explains why in lots of media whenever most Stormtrooper's are shot don't have visible or "messy" exit wounds from blaster fire like we see with some characters who are shot with blasters in certain scenes (I.E. Greedo), or why they are not blown to bits or have massive craters in their body like we also see from certain blaster shots in a few scenes when they hit walls of metal or concrete is because said armor dispersed and dissipated the blaster energy when it impacted the body armor worn by Imperial Stormtrooper's.

Thus why they just have scorch marks on their body armor, (or sometimes literally none at all), it is due to the fact that the armor worked like it was made and designed to do. So lots of the "dead" Stormtroopers aren't actually "dead" based on what's stated here.

Not to mention that the Imperial Stormtrooper corps being given such armor to protect them from various glancing and direct blaster shots makes sense as it is considered an elite branch that the Empire went and invested lots into, and members of the Imperial Army and Navy are screened for when meeting lots of specific requirements, and so are volunteers as well, along with if they successfully passed the 2-4 year long training they're given at places like the Imperial Carida Academy, with certain sub-units being given even longer training.

So they'd obviously not want to waste just valuable "assets" militarily speaking when it comes to how the Imperial Stormtrooper corps was viewed as a branch of the military as a whole.

Although the Imperial Handbook claiming that "close range" shots will penetrate the body armor is also an interesting detail as we've seen lots of cases where a lot of Stormtroopers are hit at "close range" and we do not see visible penetration and just scorch marks, or sometimes none at all even. Making me wonder if their definition of "close range" is actually different in the case of the Imperial Stormtrooper corps.

Also, the Star Wars Sourcebook Says their body armor only provides "partial protection" from Blaster Fire, yet other sources made by West Endgames say the exact opposite, and the Sourcebook made for the novel "Heir to the Empire" says it provides only partial protection from most environments, yet the Rebel SpecForce Handbook says otherwise.

Also, if the armor of the Imperial Stormtrooper corps was designed to protect against blaster energy (and it also protects from shrapnel, explosions, along with lots of kinetic weapons, kinetic attacks, bullets, etc) it makes me wonder if the same was true for the armor of Clones in the GAR who came first?

TIL According to various West Endgames Sourcebooks and the Imperial Handbook, the armor worn by members of the Imperial Stormtrooper corps is designed to protect the wearer from Blaster Bolts by specifically dispersing the energy of Blaster Bolts.