Some Wizard spells are named after the particular famous wizard who created them, like Mordekainen or Bigby. I find it hard to understand how a convention like this could come about, considering how stingy Wizards are about sharing spells.
The game has strong assumptions about how difficult it is for wizards to gain access to spells. Wizard player characters in the game are expected to slog through tombs and ruins to learn new spells, or otherwise independently discover spells on their own as they level up. If, hypothetically, a player could join a wizard organization which shares access to spells, and uses a standardized spell notation, it would break the game and make the character overpowered. They would have far greater access to the wizard spell list, and in some editions, have the ability to copy spells from other wizards in their organization at significantly reduced cost due to the standard notation.
Obviously from this we can conclude that the game intends for wizards to be unwilling to share their magic with other wizards in most circumstances.
But if Wizards are so stingy about sharing their spells, how could any spell become popular enough to be named after its author? Did the authors of these spells intentionally distribute them or something?
Best Answer
Tributes and Legacies for Characters in Olde Greyhawk
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).
Example #2:
How Drawmij's Instant Summons got named and developed. Less rigor than the magical research rules would require was behind it.
Example #3: Rary's Mnemonic Enhancer
Since you ask "what is the in game justification" the answer is either:
or
But no wizard would share magical secrets!
That assumption is not universally applied by all players, and apparently wasn't applied in the Circle of Eight.
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