Alt limits are put in place for a variety of reasons. On most game modes players are restriced to a total of 10 accounts on ManaCube at a time. Typically this would be a "main" account and 9 alternate accounts (alts).
Alts are typically used to AFK mob grinders or cactus farms to generate income. Since spawners and crops only work when the chunk is loaded, alts are able to load multiple chunks and keep these sources of income running.
WARNING: Console clients that have accounts automatically rejoin the server after a reboot are allowed. However, any type of cheat, such as illegal mods are not allowed and will result in punishment.
Alts are not allowed to be used to boost a player's position on leaderboards. Some game modes have additional rules on the use of alts. Before deciding to use an alt, please review the rules for the game mode you are playing.