WARNING: The following contains spoilers for Star Wars: Darth Vader #6 by Greg Pak, Raffaele Ienco, Neeraj Menon, and VC's Joe Caramagna, on sale now.
The Imperial Guard are red-robed warriors who stand as the Emperor's personal bodyguards, duty-bound to protect him from all threats. However, the Guard don't seem to do their jobs very well or even seem relevant compared to the power of Emperor Palpatine. But in Star Wars: Darth Vader #6, the Imperial Guards finally get a chance to demonstrate that they are much more than fancy ornamentation to make their Emperor look more imposing.
After returning to Palpatine following his quest to learn how Padme Amidala truly died, Vader was attacked by his master. Palpatine was livid that in Vader's quest for the truth, he had wandered from the dark ways of the Sith, becoming weaker and more lenient instead of more ruthless. What had started for Vader as a quest for answers and then vengeance became something else entirely, ending with Vader sparing the lives of confessed rebels.
To remind Vader of his place in the Sith hierarchy, Palpatine attacked him with his Force lightning, short-circuiting Vader's suit and severely weakening him. With that done, Palpatine ordered his Imperial Guards to attack his apprentice. Although Vader reached for his lightsaber, Palpatine disarmed him, giving the guards an opening to attack with their Force pikes. Vader, already slower due to his many cybernetic limbs, was electrocuted several more times by the guards' weapons, further compromising his suit.
However, even without his lightsaber, Vader is never defenseless. After a quick beating at the hands of the Imperial Guards, Vader reached out with the Force, telekinetically strangling them and a nearby Mas Amedda, who was foolish enough to brandish a blaster in Vader's direction. Before he could kill them though, Palpatine once again intervened, grappling Vader in a Force choke of his own and indirectly saving the lives of the guards and Grand Vizier.
This battle, while short, was still very telling of the terrifying skill the Imperial Guards hold. Palpatine is the most powerful man in the galaxy and has never needed anyone to protect him, but the guards are still capable of doing their jobs. Due to his position of power, the assumption can be made that Palpatine only keeps the Imperial Guard around as a symbol of his authority and as a way to intimidate any dissenters. But even though the Emperor could destroy most challengers with a literal flick of his wrist, he has always felt that sort of thing to be beneath him.
This is where the Imperial Guard comes in. Even though he has no need of them, their combat prowess is enough to deal with the usual upstarts that challenge the Emperor's authority. And their fight with Vader demonstrates that the guards are highly trained hand-to-hand combatants who can give even someone like Vader difficulty. And this isn't the first time that the Imperial Guard has demonstrated their talents.
In Legends Canon, Expanded Universe lore of the Imperial Guard was explored further in Star Wars: Crimson Empire by Mike Richardson, Randy Stradley, Paul Gulacy, and P. Craig Russell. The series follows Kir Kanos, a former Imperial Guard on the run from the new rulers of the Empire following Palpatine's final death. This series gave much more insight into how dangerous even a lone Imperial Guardsman can be.
As it turns out, Imperial Guards are the best soldiers from the best Imperial squads. The cream of the crop is selected and brought to an isolated planet where they train day in and day out to hone their skills. Though they are not Force wielders, their loyalty to the Emperor is absolute, even over personal attachments to friends or family and they are among the best fighters in the galaxy.