For what reasons did the Emperor Hadrian order the construct

For what reason(s) did the Emperor Hadrian order the construction of a Wall across Britain?

Scotland located north was not part of Roman Britain. After the Romans invaded southern Britain, they had to defend it. Emperor Hadrian order the construction of a Wall across Britain to defend the region from invading armies of the north. (The wall ran from Wallsend in the east to Bowness on the Solway Firth)