PPDs can be useful to printing applications. They inform the application about printer capabilities, and can be very detailed. Mine includes all the possible pre-set paper sizes (413 plus custom), printable area, colorspace (RGB), built-in fonts (85), available RAM (73M) .....
I'm OK with Foomatic dropping LPD as a built-in queue method, because the "direct" method can still be used with LPD.
CUPS and the many add-ons provide many more connectivity options and print management capabilities. But ... to get printout from an application to arrive successfully at printer is, under the covers, a little like operating a
Rube Goldberg machine. There are a vast number of moving parts. When one of those fail, it's a nightmare trying to conduct problem source identification and resolution.