Prevent spam and abuse from the end-user. To this end, gas consumed during message execution is typically priced, resulting in a fee (fees = gas * gas-prices). fees generally have to be paid by the sender of the message. Note that the BancChain does not enforce gas pricing by default, as there may be other ways to prevent spam (e.g. bandwidth schemes). Still, most applications will implement fee mechanisms to prevent spam. This is done via the AnteHandler.