Hows it possible that most game updates needs a client restart but sometimes hotfixes doesnt need a client restart.
Client restarts (coldfixes) are required whenever any client data has changed, which is almost every time we reboot the servers.
Hotfixes are purely server-only changes that take effect immediately; since no client data changes, no reload is needed.
We also have the ability to do "warmfixes" for some specific client changes. Those require a client restart to load the new version of the client, but you're still able to play in the moment.