Why did the franks raid?

From the 5th century CE onwards, as Roman power declined in northern Gaul, the Franks expanded into Belgium and northern France. Later on, the Franks resumed their expansion process and, by the first half of the 6th century CE, they gained control over part of central and southern France and a small portion of northern Spain. During their interaction with the Romans, the Franks raided the Roman Empire on numerous occasions, but some of the Franks also served as soldiers in the Roman army.

what? I don't know sorry