You will find this configuration to be the most common , so that is why if one letter is broken or died of natural causes , then there is an interruption of the flow of electricity and the sign will not work properly.
There is another configuration that you could encounter as described in the second photo in which the transformer has a midpoint connection.
Each transformer has a limited amount of neon glass that it can light up ,depending on the output voltage and the diameter of the glass.
The smaller the diameter of the glass the more voltage it will need , and also any neon that is filled with neon gas will need more voltage than the neon glass filled with Argon.
So if the sign has a lot of letters you will probably see more than one transformer , but the wiring is the same , each transformer will light up a group of letters.