Why did Viking settlers establish communities in Normandy?

Viking settlers established communities in Normandy primarily as a result of their transition from raiders to permanent inhabitants through diplomatic negotiation with the Frankish crown.

During the 9th century, Vikings frequently conducted plundering raids along the northern and western coasts of France, often using the lower Seine River valley as a base for winter quarters. As these raids grew in scale, Frankish rulers occasionally employed a defensive strategy of granting land to Viking leaders on the condition that they protect the region against further incursions by other Viking groups.123

This strategy culminated in 911 when the Frankish king Charles III the Simple signed the Treaty of Saint-Clair-sur-Epte with the Viking leader Rollo. Under this agreement, Rollo was ceded territory around the mouth of the Seine and the city of Rouen, becoming a vassal of the king and the first duke of Normandy. While Rollo’s own origins are debated, his followers were predominantly Danes who settled the region, eventually intermarrying with the local population to form a distinct culture.412