[caporaltito]

Also, if you put a guy controlling Normandy, he will control the mouth of the river Seine which runs in Paris. So you secure the biggest city of your kingdom from being raided from this river by enemies rowing it upstream, which was the biggest problem with Vikings raiders back then.

[mytailorisrich]

I'm thinking the King thought Rollo was a massive pain in the ass but also a very capable guy so decided to kill two birds with one stone.