I really like being able to switch instances when, for example, notorious gathering bot players are active and I tire of seeing that train of characters following the same flight path and hitting nodes in unison.
So staying in the same field zone but changing instances is really nice to escape certain immersion-breakers or unwelcome people you might say. Also useful for questing to avoid massive crowds when new content is added, especially the type of quests that spawn monsters into the field.
The ideal would be removing world-specific instances and just have a unified block of datacenter-wide instances for field zones. So you wouldn't need to use world visit feature at all for field zones and just have access to the same selection of field instances regardless of what world you're currently on.