The short reason you should always charge the battery to maximum capacity before use is - because the last stage of the charging cycle is to balance the voltage between the internal cells. The LiPo battery has internal cells. While you are using the battery - it may discharge one cell faster than another.
The voltage between internal cells must be balanced before EACH use - and the way to do this is to charge the battery to full before each use - even if you only used it a little bit the time before.
For this reason, it is not recommended to use your battery more than once before recharging it, because you run the risk of over-discharging one or more cells - but you won't know this because another cell has more voltage so the total is still enough to power the drive - but in doing so - it will damage the battery.
If you don't use it every day, charging gets even more complicated. See the charging in formation for your battery by clicking the appropriate link below:
Charging Home Original 11.1v battery
Charging Home Plus 14.8v Battery
LiPo batteries do not need to be fully discharged before recharging, see: Recharge Before Fully Discharging - YES!