In order for an action to be fulfilled, GAS will first calculate the amount of processing power that is needed for the given action to be executed. Every action requires a different amount of gas, some more than others. Creating a smart contract will require more gas rather than making a small purchase on the blockchain. 

Miners are encouraged to pick the transaction with the highest gas price first. This is because the amount of work a miner needs to do will always remain the same however depending on the amount of gas that has been paid they can earn themselves a better price. Gas price equals transaction speed, so in order to have your transaction fulfilled before others you will need to ‘outbid’ the current gas price. Reasons to pay more gas for a transaction could be due to the urgency of the transaction