Pre 1.6 randomness wasn't truly random and could be predicted. The most commonly used was clay farming where if you know the pattern you can make a ton of money early by getting clay on the beach and selling it to Robin. Apparently you could also exploit crab pots and predict mushroom floors, but I learned about that 2 minutes ago, so you'd need someone else to actually explain it.
Now that simply doesn't work. Clay doesn't spawn in a hidden pattern that shifts a tile every time you till the ground, its an actual random drop. Turning on legacy randomization brings the old system back. Useful if you're planning a speedrun or something similar. Otherwise not much reason to use it for what I know.