When you break a shadow orb there is a chance for a meteor that night I believe. Last time I went into a new world I made I went straight to the corruption and broke two shadow orbs right before nightfall, actually got two meteors to fall.
If you smash the orb at night you have a 50% chance of it falling instantly. If you smash it at day you have a 50% chance of it falling that night.
Anyways manders, his point is there are only X number of shadow orbs that generate on the creation of one map. So he creates new one's so he can essential get meteorite ore. Which is needed for ammo later if you use the space gun or whatever its called.
And shadow orbs are the little things that spawn in the corruption every three of them you kill summons one of the bosses eater of worlds or somthing like that, you kill him get a crap ton of demonite and shadow scales.
A few other reasons to start new maps. -Keep a map out of hard mode. While having a separate one for hard mode. -Starting new character's with friends who just got the game. Which sucks till you get a new grappling hook then its fun again.
My friend also had no hell forge on his map. Thought he was just being dumb and couldn't find one but after 7 hrs of looking he didn't have a single one. Which his map was only a M size instead of L. So his ow fault. -Have a map that has a massive farm for potion ingredients. For example I started a glowing mushroom farm today to help me make a massive amount of greater healing potions to bring into my hard mode map. Reason being I didn't do this in hard mode is because a clown spawned in my farm and threw a bunch of bombs and blew it to smithereens.
-You won't necessarily get every loot item in the game on one map generation -^You might not even get a shadow key. -^^In my first world I only had three shadow chests, luckily the first one had the fire flower, but none the less that doesn't mean everyone is going to find it on their first map.
Well, worlds have limited resources. Sometimes you won't find what you want in the underworld shadow chests or floating island chests. Maybe some people ran out of ore to mine. I made another world because I wanted to live in a snowy biome (corruption and hallow can't spread there plus it's cool)