The reason for charging at 1c is when your battery is new it contains some chemicals to keep the battery in a stored state.
Charging and discharging at a low rate is best to help the chemicals break down in first 3/5 cycles. Charging at 1c will also help with the life of the battery and it will last a little longer when flying(discharging).
You can charge your battery at a higher rating as long as your battery is safe to do so but I have always charged my lipos at 1c .
There is a better write up on here with more information that will help you.