Firstly, you could name the space savings. As every single file always consists of a multiple of the block size of the current File System, on average half a block is thrown away at each file too. On certain operating systems, an entire block is needed for every directory entry also. That means, that per file a waste of one and a half blocks is to be expected. In case of about 100 modules per library, approximately 150 blocks are saved in this manner.
But there are other advantages. There are the better overview and also the faster access just to name a few. As you mostly need several fonts of one reslution, you only need to open one file, and read the directory there. After that every access to a font is only an ``fseek'' command. As this way for each resolution only one file has to be opened, you can keep this open all the time the program is running. Thus you avoid a constant opening and closing of files.
The Flib program is however not only specialized in PK-Fonts. In theory you can archive every type of file in a library. As the program source is available for the Flibprogram, other programs could be adjusted to it, who load a lot of help files.