Stormtroopers are supposed to be Star Wars' frighteningly efficient and merciless imperial soldiers, but they're famously bad shots and their armor doesn't seem to protect them at all. In Star Wars: Episode IV - A New Hope, they seem to be a fearsome force early on, but once Luke, Han, and Leia begin to fight back, stormtroopers suddenly seem far less intimidating. All it takes is one blaster bolt to kill fully-armored stormtroopers in the original trilogy.
The Imperial troops become even less threatening in Return of the Jedi, where the diminutive Ewoks overpower the armored stormtroopers with primitive weapons ranging from spears to rocks. Rogue One: A Star Wars Story doesn’t help matters by showing stormtroopers being incapacitated by a seemingly simple wooden stick. With all of this in mind, it’s understandable to ask “Does stormtrooper armor actually provide protection?”
According to Ultimate Star Wars, New Edition from the current canon, stormtrooper armor is impervious to projectile weapons and most melee attacks. The armor also protects the trooper from blaster bolts by dispersing the heat from the bolt, often leaving them incapacitated (but alive) or entirely unimpeded. This still doesn’t explain why a single blaster bolt in the films punches holes through the armor. The answer to that question has less to do with the armor than it does the blasters themselves.
Standard Rebel weapons are described as being able to pierce armor, even on their lowest power settings. Because of their effectiveness against stormtroopers, the Empire made them illegal to own. Imperial blasters are also more than capable of punching through stormtrooper armor. Common civilian blasters, however, are supposedly unable to kill an armored stormtrooper without multiple direct hits, however these weapons are conveniently not used much in the films.
As for stormtrooper armor’s apparent ineffectiveness against melee attacks in the films, certain sentient species, such as Wookiees and Lasats (such as Zeb from Star Wars Rebels), may have the natural physical strength to incapacitate or kill an armored stormtrooper. Though small, Ewoks are likely able to harm armored stormtroopers through sheer strength alone. Moreover, the Ewoks’ primitive weapons can still stroke at the gaps in stormtroopers’ armor in Return of the Jedi.
Of course, while there may be in-universe explanations for why stormtrooper armor isn't always as effective as marketed, there's also the fact that the movie plots ultimately need the heroes to cut down stormtroopers quickly. Escaping from the Death Star is already an impossible enough feat without truly blaster resistant stormtrooper armor. The Imperial military happens to be at war with a faction that exploits all of their weaknesses. The soldiers and allies of the Rebellion and, of course, the heroes of the Star Wars original trilogy are simply well-equipped (or strong enough) to bypass their protective suits.