A coverlet is a denser weight sheet, it mainly made from absorbent material. so you have choose to use it either winter season or summer season. Fabric of coverlet contains cotton threads inside which it is likely providing his two options in uses.
In summer, It is a lightweight material than blanket, you can sleep with more comfortable and happy.
In winter, It helps the blanket or duvet to provide more heating to your body to take deeper sleep.