Measuring cups are not strictly necessary, but they makes cooking easier. Especially if you are using recipes from around the world, with both metric and imperial measurements.
It is nice to have a measuring cup that is just the right size.
These measuring cups, sold in a set of four with the following measures and conversions printed on the cups:
1/4 cup = 60ml = 2fl oz
1/3 cup = 80ml = 3fl oz
1/2 cup = 125ml = 4fl oz
1 cup = 250ml = 9fl oz
I prefer the stainless steel cups