Really you have to consider "Viking raids" in terms of three broad categories; the piecemeal raids of the late 8th and early 9th centuries, the organised large-scale expeditions of the mid-9th century, and the strategic raids from the 860s onwards.

The raids which begin in England with the attack on Lindisfarne in 793 are originally based on targets of opportunity, simply striking at isolated communities, particularly coastal religious communties. It's later into the 9th century when raids become much larger and organised that more care in target selection becomes evident. The raids which hit London in 841 and 850 clearly target it because it's a major wic site, for example. But most often it's not raiding that's profitable, but not raiding. The Anglo-Saxon Chronicle makes many references to regional populations 'making terms' with raiders or otherwise paying them off. Raiding is most effective as a form of demands with menaces, and indeed during the reign of Æthelred II in the early 11th century, the amount of money being paid to assuage raiding on England reaches frankly staggering amounts.

By the 860s, when the 'Great Army' arrives in England, raiding is more of a strategic imperative, desiged to tie down large amounts of manpower and prevent an effective response to the main Danish invasions. Asser in his Vita Ælfredi and the Anglo-Saxon Chronicle for 871 both record the huge numbers of battles fought by the West Saxons and that the need to fend off myriad raids as well as fielding the main army was causing severe manpower issues, a problem that was only solved by the establishment of the network of burghal defensive hubs and local strongholds which formed the basis of successful English strategy for the next 70 years.