What command 'expo start -c' actually do?

Running expo with the command expo start -c clears the cache.

https://docs.expo.io/versions/latest/introduction/already-used-react-native#useful-commands

Although it is not clearly explained here you can see that the following command takes the -c flag

expo start -c --localhost --ios

Their explanation is

start expo server, clear cache, load only on localhost and open on iOS simulator

So following the order of the flags it is clear that the -c flag clears the cache. That is why your errors will disappear as the cache has been cleaned.