The thermal efficiency measures the ratio of work output to heat input (fuel supplied).
The higher value of thermal efficiency signifies that the for same value work output the fuel consumption is lower (or in other words it can be said that for same amount of fuel supplied the work output is higher)
The more the fuel consumption by any system the will be the operation cost of the system. The amount of fuel is not infinite and it takes lot of time, effort, energy, man power, and thinking to find, extract and process the fuels. So, the cost of fuels keep increasing and the amount keeps depleting.
The main reason to have better and more efficient system is related to economic and environmental/ecological aspects along with thermodynamic aspect.