1. I just want to know if they are already getting reward in the form of bitcoin they why there is additional fee we have to pay for every transaction?
More transactions => larger data size => more workload. Imo, when the network is congested and over-loaded, miners have to work extra hard to include each of those transactions in the block and for that we need to pay more fees to help them cover the cost.
Fees are not necessary for the processing of the transaction on Bloakchcain
Do you know that after bitcoin reaches 21million circulating supply, miners can only earn from the transaction fees we pay to them? Fees gives a good incentive for them to work for us and keep the network stable.