Well think of it this way, when a Pokémon is captured/hatched it is registered to it's trainer. Evidently when a Pokémon is taken to the Pokémon Center or stored in the PC data is collected on that trainer. This is probably the reason why a trade machine is needed to trade Pokémon between trainers in the Anime, as if two trainers simply exchanged Pokémon without using the machine the data on that trainer would be outdated. The data collected on trainer's is very important in the Anime, as trainers can use this data to gain information and assess other competitors in tournaments. In Episode 266 of the English Dub (titled - A Claim to Flame!), Misty says "we can get data on the other competitors using this computer". Ash later uses the computer to gain information on each of his opponents and to analyse the data collected on them. Another reason why the trade machine is necessary to trade Pokémon in the Anime could be because if you were to trade Pokémon without using the trade machine it perhaps would not register the traded Pokémon in your Pokédex.
Additional Information:
>Despite it being one of the key aspects of the games, trading is exceptionally rare in the anime. Only a few trades have been shown taking place, and only four of them thus far have been truly significant. Fans have often speculated about the reason for this. Rather than simply exchanging Pokémon, most trades are carried out by placing Poké Balls into a specialized trading machine, with a monitor that displays silhouettes of the two Pokémon as they pass each other.
- In Battle Aboard the St. Anne, Ash traded his Butterfree for a Gentleman's Raticate. He wasn't happy with the trade, however, so he traded it back by the end of the episode.
- In Tricks of the Trade, Jessie unintentionally traded her Lickitung for Benny's Wobbuffet.
- In Here's Lookin' at You, Elekid, James is forced by Jessie to trade his Victreebel for the Magikarp salesman's Weepinbell.
- In Throwing the Track Switch, Ash traded his Aipom for Dawn's Buizel.
- In Evolution Exchange Excitement!, Bianca and Professor Juniper trade their Shelmet and Karrablast so that they could evolve.
>The trade machine used in the anime has seemingly been adapted into the game canon, with Professor Oak's laboratory in both Pokémon Stadium and Pokémon Stadium 2 having this type of machine for trades. In the same way, when link cables were finally able to accept the data of two Pokémon being sent at once (rather than one at a time as in Generations I and II), the Pokémon are shown passing each other in the link space.
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