The lower gear (higher number) allows the engine to run at a higher RPM at a given road speed and, importantly, to gain RPM quicker when starting off.
I other words, when you are driving 55 mph, the engine RPM will be higher with a 3.73 than with a 3.55, and evev higher with a 4.10. This higher engine speed lets the engine operate in it's power range, rather than 'lugging' at low RPM where it can't develop power.
Starting from a standstill, the lower gear lets the engine rev quicker to it's power range and lets you get moving easier and quicker.
Since the engine is running faster with lower gearing, it uses more fuel. "Lugging' (straining, or over working an engine at low RPM) will also cause an engine to use more fuel.