Armor, traditionally here on Earth anyway, has never been a foolproof way of keeping the wearer alive, but it's helped. Body armor on today's modern soldier for instance can stop a weak shot, deflected shot, glancing shot, etc... It also can protect against some shrapnel and other combat hazards.
In World War I, Trench Raiders often donned armor because of close-quarters combat situations that were fairly routine, but they obviously wouldn't protect the wearer perfectly.
Going all the way back to the Middle Ages and the advent of plate armor, the plate armor was quite protective against long bows, and it certainly helped protect the wearer against some melee combat, but the crossbow put an end to that, and finding the chinks in the armor became the norm for the common footsoldier... Weapons specially designed to draw the armored soldier off his horse and then systematically hack/slash at him till you opened up his shiny duds was the rule on the battlefield... And then there was the ever-present "blunt force trama" where if you hit someone hard enough with a club or heavy weapon, even in armor, you still break something.
I think in Star Wars terms the armor did what armor's done for centuries on Earth... It gave some protection, but certainly not complete protection. In Star Wars blasters pierce through it with ease but even in Episode 3 we don't see a ton of heavy combat up close save for Utapau which seems to not have things like Artillery and whatnot taking place to see how the armor would react to a fragmented energy blast in a cluster of troops... Would a couple die, and a couple get up? I think so personally... Some would get the lucky glancing blast while others took a full-on hit and died. But would ANY have survived if they didn't have the armor on?
On Endor, the same thing applies as in the middle ages when a knight would be unhorsed... A warhammer to the head could knock him unconscious at the very least, despite his helmet's supposed protection. A bow hit to the armpit, or the groin, would incapacitate him for certain... A broken arm at the elbow, a broken knee or ankle, a knife in the torso between the plates... Stormtroopers were swarmed by Ewoks, having 3 or 4 weighting them down then beating them mercilessly... Not a positive situation despite the armor.