They're by no means equally compelling. But they are viable ways to generate currency, you progress in them over time as a specialist, they feed back into the player economy performing tasks that other people want performed, and they are, importantly, in the same world, on the same shard. I know not to go near Orc Camp because there's a group of player killers down there, despite the fact that there's a rich Agapite vein running through the mountains near the entrance that I would love to mine and make armor out of. Back to the relative safety of Minoc for me, however crowded. In some timeline two weeks in the future, I band together with a bunch of other players (most of whom just want to farm orcs) to kick them out. Territorial control, even without any formal mechanics of territorial control, is closely correlated with narrative and socialization; I wouldn't have met any of those players if we were all on our own separate instance.
Eve Online accomplished something a little more combat-focused, but similarly diverse in playstyle, mostly by dint of having a single large persistent world-shard with minimal functional instancing.
The concepts come with the idea of a persistent game map, which is online 23/7 and never wipes.
When UO came out, it had to deal with a large game world that nevertheless would be too small to reasonably hold a number of players that represented financial success. So the game world was duplicated on multiple servers running the same map. A player on Atlantic server could have no interaction with a player on Chesapeake server. If you wanted to play together you needed to make characters on the same server. In-game this was reflected in cosmological lore about the world being some kind of crystal that was shattered into shards. This jargon spread, to some extent, to be a generic terminology in MMORPGs.
I believe UO launched with only three shards, and added a couple dozen at the height of its popularity.
Eve is notoriously a single-shard game, with the player count being accommodated in other ways as the map grows larger and more organized play is added patch to patch, and the actual play changing notably from the era of 5,000 simultaneous online to the era of 50,000 simultaneous online.
Even without deep skill-stat specialization, when your alliance has 500 people it lends itself strongly to role specialization, because an alliance of 500 people is an organization of 500 people, who find different things fun in different contexts, and whose operations have every category of need you would find in a small nation state, from diplomacy to collective resource allocation to logistics to surveying to espionage to military service, recruiting, communications, practice and command.
Eve has every player on the same global system, and "shards" only by running the code for a particular system on a particular server. As a result, if you're in Eve Online every other player has some influence on you however minute.
Eve Online accomplished something a little more combat-focused, but similarly diverse in playstyle, mostly by dint of having a single large persistent world-shard with minimal functional instancing.