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

I know, but don't tms pass down like breeding moves? Read the entire question!

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I did. If you have used it on another Pokémon, it is not always possible to chain breed the tm to another Pokémon. Therefore, you can breed it if it is another species.

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Rayannelle is right.  This is why the TM Egg Moves started prior to Gen 5.  It is no longer relevant in Gen 6, and Gamefreak decided not to change it.  Since your question doesn't specify Gen 6 and doesn't have a Gen-6 tag, this answer is 100% correct.

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But Mandibuzz is a gen 5 Pokemon, so your explanation wouldn't make sense. Besides, don't moves learned by level up also pass down if the child can learn it as a TM?

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This feature was implemented BEFORE Gen V. After gen V, they simply decided not to change it.

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What Ray said. "This is why the TM Egg Moves started PRIOR to Gen 5." You must have just overlooked it.

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Don't moves learned by the father by level up also pass down if the child can learn it as a TM?

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That's the point.  Prior to Gen 5 we bred our Pokemon with TM moves so that it would be easier to get a Pokemon with a TM move.  EQ has always been massively popular, but some Pokemon can only learn EQ via TM/breeding.  In Gen 4 TMs were consumable items, so unless you could trade for an EQ TM or find one with Pickup, then you had to breed.  

This question is already 99% obsolete in Gen 5+6, once Gen 4 becomes so old that nobody does competitive, then this will turn into a Pokemon trivia fact.

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But since that's true, any TM move is automatically an egg move, unless the Pokemon is all male or genderless. What's the point in having them noted separately?

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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.