>>28103261>>28103277>>28103304>>28103339It was to do with the fact that you could easily cause Magnemite to have a very high encounter rate, very early in the game without having to search for any other Pokemon or items.
Since Mags have the ability Magnet Pull, which attracts other steel-types in the field, once you've hunted around a bit to grab one it's very simple to get others. Since they appear frequently anyway in Virbank (second gym city) you can just easily catch loads even if you have access to nothing else.
As for why trading Magnemites helps at all, each Mag you trade gives you another unique ID, and the more you have the more Join Avenue grows. Once the Magnemite meme had built up online, people knew to look for Magnemite if they wanted to level up their Avenue, and there was a healthy supply of them on the GTS to choose from.
