The act of balance charging is leaving the device plugged into its charger for an extended period of time (1-3 days) This gives the charger/BMS a chance to fix any slight voltage differences between cell groups. A balanced battery will have more cycle life and have more run time per cycle.
Balance charging can be a preventative measure - it keeps the cells more in-line long term and helps counter imbalance issues before they even happen. How often you should balance charge can really depend on the use of the battery, but in general we recommend once every 5-10 cycles as a minimum. It is done balance charging when all the cell Voltages are even.