Cassettes for single chainrings have a wide gear range. So you benefit from a high chain tension and a chain that doesn’t come off. Getting rid of the chainring, front derailleur and shifting lever saves a huge amount of weight too.
You can cover a very wide range of gears with two chainrings – which is ideal when you’re riding up and down lots of steep slopes. The easy-pedal gear allows you to save your strength going uphill, but you also have a gear for pedalling downhill too.
The more chainrings, the more gears. But what matters is the range of easy-pedal to hard-pedal gears.