What's the point in having one Pokemon being able to learn a move as a TM and an egg move?

Question

I noticed that Mandibuzz can learn roost as both an egg move and a TM move. Since TM moves pass down the same way as egg moves, doesn't that mean that any TM move is automatically and egg move? (unless the Pokemon is 100% male or genderless)
Again, my source is the Pokemondb page, so was there an error?

Comments

No error.  It happens, it's useful, it does have a practical purpose for people who haven't finished gameplay.  Beyond that I can't think of any competitive benefit that applies to anything.  Just Gamefreak being Gamefreak.

Answer 1

In all the cases I know, including roost, the move stopped being on a TM in a later generation. So some of the Pokemon that previously learned the TM get the egg move in the later generation.
source

Answer 2

Prior to Gen V, tms were once-use. Therefore, you could breed them if you had used the tm.

Comments

Because there are two different ways to get that move on the Pokémon. If you didn't put it under TMs AND Breeding, people might not know that u can get it both ways.

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No, but then it would only make sense to put every compatible tm move under egg moves.

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But not every TM can be passed down as an Egg Move :P

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Really? why?

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Okay, hopefully I got better at explaining myself during these past 4 years.

I'm going to use Farfetch'd because Vullaby is a bad example. In generation 5, Farfetch'd can know roost as an egg move. So if I bred a female Farfetch'd with a male Chatot that knew roost, then the eggs would be Farfetch'ds that know roost, right?

If roost was a TM move for Farfetch'd, instead of an egg move, then Farfetch'd should still be able to inherit roost from any father that knows roost. So if I bred a female Farfetch'd and a male Chatot, then the children will know roost as long as the father knows roost. The father could have learned roost by level up, TM, or even passed down from its father. IT DOES NOT MATTER. The children will know roost.

In general, any move that is on a TM can be passed down the same way as egg moves. It doesn't matter whether the father actually used the TM to learn the move.

So listing a move as an egg move has no in-game effect if it's already a TM move.