A Horde of New 5th Edition Monsters! DM: “A collection of hundreds of eyes floats down the corridor toward you, trailing ganglia and dripping caustic fluid that sizzles when it hits the ground. What do you do?” PLAYER: “I retire, and become a farmer.” Whether you need dungeon vermin or a...
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Use this book without using the PHB, DMG, and MM as a person who doesn't have the core rules and may be entirely new to the game, seeing as how most of the populace has never played D&D in their lives.
Also, the very name "Tome of Beasts for 5th Edition" directs people to buy the 5e core books.
Like I get what you're saying. As someone who already owns those three, I don't need 'em. And someone using the SRD alone wouldn't need them, either. But they are -incentivized- to make that initial purchase. Which puts money in WotC's pocket. And is likely as not to lead them to buy more WotC products because they're the "Official" material.