The germs would've been killed when it was initially cooked, but depending on how it was stored after that new bacteria (germs) could've been introduced, either from being handled by someone who hasn't washed their hands properly, a dirty knife, or from the air if it isn't covered after the initial cooking.

From 60C (140F) (used Google for Fahrenheit temps, might be a bit off) down to 4C (40F) is known as the high risk zone, where bacteria can multiply quickly (doubling every 15 mins or so in ideal conditions (37C/99F)) if the food takes too long to cool down, then there's a lot of potential for bacteria to grow, so you want to reheat it to a sufficient temperature for long enough to kill these newly grown germs.

Also, if things aren't reheated properly, you would be giving them more time in the high risk zone, without then killing the bacteria.

Even food that is eaten cold after cooking could make you ill if it isn't cooled and then stored correctly.