There must be a line in the sand that prevents players from joining the server that shouldnt. Whitelists are easy to maintain by either yourself or a trusted moderating group and can be turned on or off at will. Allowing everyone access can create bigger communities faster than adding one name at a time to whitelists. Players that join and are interested in causing problems can be banned either temporarily or permanently by using administrative commands. A server owner can make great use of plugins to control customized chat and other responses to people who would otherwise be allowed to participate but may say the occasional naughty word.”]
Source: https://gbhackers.com/securing-your-minecraft-server-to-protect-from-cyber-attack/