What are the use cases for a closed solder jumper on a PCB?

In this case write protect is optional. And disabled by default.
Cut the trace if you want to enable it, link by solder to disable it.

Sometimes you have a need for normally closed solder jumpers.

Sparkfun: How to Work with Jumper Pads and PCB Traces

In addition to the specific explanation that @Jeroen3 provided for your example, there is at least one additional purpose of this type of jumper (Note that the symbol can also be reused for more general jumper purposes and not only the solder-jumper footprint)

Serve as a "net-tie" (Term from altium).

If you have say, AGND and DGND , or GND and SHIELD and want to make certain that they are connected together at exactly one point (and not common net which prevents you from using DRC to review the ground separation), you may use a closed solder jumper to do this if the EDA does not have a special element.