What is PPM?
PPM is a Netpbm image: a tiny text header giving the dimensions, then raw pixel values with no compression whatsoever. Almost nothing displays it, which is rather the point - it is what image-processing code and academic tools emit when they want to be trivially readable.
What is BMP?
BMP is the Windows bitmap: pixel values written out with essentially no compression, which makes the files enormous and the format trivial to read. It survives in Windows tooling, embedded systems and anywhere a program wants image data without a decoder in front of it. Rows are stored bottom-up.