We use mining in order to do the following;

• Check and uphold the integrity of the blockchain
• Mining is also the only known means to validate transactions on the blockchain. This process takes alot of computational power and energy to be able to do work (proof of validation). So mining is need for a PoW system
• Miners also make it possible for all the bitcoin transaction to be included in the blockchain and remain immutable
• Finally we mine in order to have bitcoin rewards as incentives and also earn transaction fee

 In a layman's way, is to keep the bitcoin network secured. Mining becomes more or less difficult depending on the number of miners and their hashpower. If there's no difficulty in mining I think 51% attack would have happened since.