A masked ROM can be thought of as a factory programmed part.

The advantages are a lower production cost and additional customer savings by not having to program the part themselves.

The disadvantages are a one time mask charge of a few thousand dollars, and of course the inability to change the code.

Economically, you need to buy >5K units to achieve any real cost savings, since the unit savings need to be greater than the mask charge.

There are different ways to make a masked ROM. Basically, you have to remove a transistor to "program" it. Some vendors use an implant to destroy the programmed transistors. Others short it out with metal.