The point is surely to get another chance to grill the immigrant and potentially bar them entry. Other countries don’t always do this: Canada for example has inland changes of status and I believe renewals.
For many visas, people are expected to maintain a home that they intend to return to in their country of origin. Forcing people to leave every once in a while makes this more likely (at least in theory.)
Lack of real domestic enforcement would be an obvious one, so everything gets pushed to the border entry points.
Wait, what’s getting enforced here, anyway? They are getting a new visa, no? I don’t believe this rationale at all.
See the recent events and pushback on immigration enforcement, to answer this, white whole cities (with their c judicial system and and police included) refusing to participate, and governors condemning enforcement.
Most visas don't permit their holder to work.
> Wait, what’s getting enforced here, anyway?
The visa holder is forced to expend nominal financial resources and time in their home country (where they agreed they would return to.) Most people can't afford to maintain a facade of residency or two actual residences (and those who can aren't the intended targets of this type of restriction.)
However, the in-country processing takes 6 months while the flagpole option (If you’re eligible) takes 15 minutes.
Most visas are for temporary guests: obtaining a visa doesn't normally give someone the right to permanently reside, and most don't permit their holders to work.