The aspect ratio defines that for any set number of horizontal units of measurement, there will be a specific number of corresponding vertical units of measure that are equal.
In other words, a 4:3 aspect ratio means that for every 4 units of horizontal measurement, there will be 3 equal units of vertical measurement.
With a 16:9 aspect ratio, for every 16 units of horizontal measurement, there will be 9 equal units of vertical measurement.
Using this logic, a 1:1 aspect ratio would be squared, whereas a 2.35:1 screen would be 2.35 times wider than it is high.
All of them have a valid purpose.