Because Pidgeys only take 12 candy to evolve (same with weedle and caterpie). So lets say you've caught 100 pidgeys (not unrealistic at all, since you see 20+ a day). Thats 100 pidgeys, minimum of 300 candies. You'll also get a candy for every pidgeotto you transfer after the evolution trick, so we can pencil in 400 candies. You get 500 experience every time you evolve a pokemon. If you pop a lucky egg you get double experience for 30 minutes. So if you hoard pidgeys, caterpies, and weedles, pop a lucky egg, then spend all 30 minutes evolving them, youll get 1000xp per evolution (500 x 2) and you can do two evolutions a minute assuming you have the pokemon and candies (not hard at all if you just save up for 3-4 days). So thats 1000 x 2 x 30 for 60,000 experience points just sitting still in thirty minutes. So people like to pair it with insence and lures so you can get double xp for pokemon you catch and spinning the pokestop as well.