Why? Font files are loaded early during boot (by vshbridge and SceFnt ). If you replace a .pgf file with an incompatible or incorrectly signed package, the system may:
Occasionally, a failed firmware update or a botched homebrew installation can corrupt the system font partition. In these cases, the system might boot, but all text appears as boxes or random symbols. Flashing a fresh firmware font package is the primary method for restoring the system to working order without reinstalling the entire firmware. ps vita firmware font package
Just remember the golden rules: