Tributes and Legacies for Characters in Olde Greyhawk
I find it hard to understand how a convention like this could come
Background on Named Spells
Named spells were first published in books in 1e AD&D.
The original spell list published in Men and Magic (1974, OD&D, TSR, p. 21) had no named spells. All spells titles were descriptive: Sleep, Water Breathing, Pass-Wall, Contact Higher Plane, etc. Greyhawk added 7th through 9th level spells, but named spells were absent even though other spells were being developed / researched / play tested or otherwise tried out. It is worth noting that in the game's formative years, classes, spells, and items were in a state of continual development.
The rules for Magical Spell research were included in page 34 of Men and Magic. Anyone could create a new spell via this means if the time, gold, and effort were put into it. (Balancing new spells was left to a given DM ...)
How did these names spells get into the book? The simple answer is that Gary Gygax played or played with the characters in question before AD&D 1e was published.
The history of Bigby and Mordenkainen or Drawmij you can read on Wikipedia, in interviews with Gary Gygax, or in back issues of Dragon magazine. You will note that not all of the spells attributed to these characters made it into published material.
Why did they choose to name the spells after themselves?
In game, those wizards were on the Council of Eight, the most famous wizards in the Greyhawk campaign setting. In character, it makes role-playing sense that fame and ego could combine to induce those wizards to name spells that they developed after themselves.
OOC, it makes sense that Gary Gygax assigned those names as a tribute to the characters. It is far more likely that the OOC justification is what led to the names.
Example #1: (From the Greyhawk tribute link).
Nystul's Magic Aura. Per Gary Gygax on Dragonsfoot: "Nystul is the surname of a stage magician, Brad Nystul, who suggested the magical aura spell to me. The brothers Mike and ? Nystul played in Len Lakofka's Lendore Island campaign"
How Drawmij's Instant Summons got named and developed. Less rigor than the magical research rules would require was behind it.
By Ward's own account, the spell originated during a session in Gygax's original Greyhawk campaign during which the players were stranded in a dungeon; Ward's character owned a magical item which would have rescued the party, but had left it in an inn before setting out. Ward remarked to Gygax that wizards should have access to a spell which allowed them to recall any item in their possession to their hand; Gygax promptly devised instant summons, which did exactly that.
Example #3: Rary's Mnemonic Enhancer
Rary was a low-level wizard created by Brian Blume and played only until he reached 3rd-level, at which point Blume retired him, having reached his objective, which was to be able to introduce his character as "Medium Rary". Gygax borrowed the name for the spells Rary's Mnemonic Enhancer and Rary's Telepathic Bond. Ironically, the original Rary was never powerful enough to cast either of "his" spells.
Since you ask "what is the in game justification" the answer is either:
- There isn't one (most likely)
- The most famous magicians in the World of Greyhawk had big enough egos to want their uniquely researched and developed spells to carry their name forward, as a legacy, long after they were dead. (plausible)
But no wizard would share magical secrets!
I find it hard to understand how a convention like this could come about, considering how stingy Wizards are about sharing spells.
That assumption is not universally applied by all players, and apparently wasn't applied in the Circle of Eight.
it would break the game and make the character overpowered
You might want to show your work in supporting that statement. The wizard is still limited by how many spells he can carry with him, and the chance that his books could be stolen or destroyed.
As to why Gygax chose name spells for his own characters, it appears to be a tribute to a major influence on OD&D wizard magic system: Jack Vance. See a spell called Phandal’s Mantle of Stealth in his story -Turjan of Miir