Plugins are used by romcenter to calulate the file crc and size. Its main work is to recognize the rom format, skip header datas and process roms datas so that two roms of a same game in different format return only one size and crc.
Here is an example of three roms with different crc and different size:
With the standart plugin, the crc is calculated on the whole files, including the header.
Now, if we switch to a dedicated plugin (here, nes.dll), the nes format is understood (ffe, nes, unf) and the file appears to be the same game.
Using plugins, you only need to store one crc and one size in the datafile to recognize all formats.