The function was useful during early OOB creation and would be again if someone decided they wanted to convert the Hungarian OOB unit names (for example ) to English
It works exactly as it was designed to BUT.....there are slight differences between the WW2 MOBHack which does have the sub-screen check box and the MBT MOBHack which does not. You have to change the entire name in MBT whereas WW2 will allow you to change part of it.
In the WW2 Hungarian example, you could change all "Lovasz Raj" to "Rifle Raj" by using the substrings control to change Lovasz to Rifle but in MBT it would have to be a full Lovasz Raj => Rifle Raj and IIRC the substring control was left out of MBT because it wasn't really being used in WW2
But if you wanted to rename all units named Rifle Squad in an MBT OOB to Rifle Section it will do it but it has to be the full name to the full name.... no partials in MBT.
That section of the MBT MOBHack Help file is a copy of the WW2 because, as Andy said, it's been so long since we used it we forgot they were different.
That said, it's nice to know somebody reads the help files and I will be correcting the MBT version of that section to reflect it's reality