"The Starswirl section is the most heavily guarded part of the archive."
If Starswirl is actually Discord, he probably messed with extremely powerful and dangerous spells, such as time travel and Omniomorphic. The Starswirl section is the most guarded because it contains all of the most dangerous spells, all designed by Starswirl (Discord)
Starswirl the Bearded was corrupted by dark magic and he became Discord, just like how Luna became Nightmare Moon.
Starswirl was Celestia's first protege. Like Twilight, he had a powerful grip over magic within himself.
Slowly went mad as he hermitted himself away, doing nothing but studying and working.
Celestia imprisons him in stone, not destroying him in the hope he could be saved one day.
Twilight reminds Celestia much of Starswirl, and for that reason she makes one big change.
She requires Twilight to make friends, hoping it can serve as a way to prevent what happened to Starswirl from befalling her.