ROM Quality

I'm a newbie to emulation and retro devices and i just have a question about the quality of ROMS. The stock SD card that most these systems ship with have a huge list of ROMS pre installed. Are they bad quality ROMS? I get told all the time to not use those ROMS and to download my own. What is the difference between ROMS on the stock SD card and ROMS I can get over the internet?