Originally posted by canned ham 91:

ose all his XP and the equipment he has in his backpack?

Nope. People commonly shift between maps and use them like "portal worlds" if you will. You can even use the same seed now with a different name for it (clone it if you will). Anything in inventory stays in inventory. This is how some people move large amounts or ore/metal bars to their home base since portals cant let metals through(they move to a chest on one map, move to their base on the first map, then go back to the 2nd map's chest with metal and hop back to the first at their base.)

I personally have lots of seeds selected for doing specific boss fights as boss farming maps. A main world map, etc. Some server owners have a main building map and then 1-2 resource maps that they replace every few weeks when resources start to get low for people.

Also remember when devs add new features and assets to biomes that an old map that already had such a biome explored will not have those new assets necessarily appear(this occurred with the mountain biomes and ice caves a couple of months ago) So having new seeds/seed clone can help with that.