Historically speaking, raiding for slaves and gold was the #1 source of income for the "Sea Kings". They depopulated Ireland and much of the English and Welsh coast through slave raids.
These Norsemen barely collected taxes as they had no inkling about bureaucracy, their income was all from raiding.
In the game though, raiding is not a viable strategy for a number of reasons:
1. Raiding means you're going to be at war with a most of the British Isles pretty soon. Alliances and vassal relationships mean that by the mid game, all the surviving factions are part of larger alliances who will all declare war on you if you raid their settlements. You get a few thousand gold from the raid but in a few turns, you have multiple 20-stack armies coming for your throat.
2. You can't raid very far. Since every province has 1 walled settlement, and you can't take on reinforcements through raiding, your raiding army will be out of steam in a few turns. if you try raiding along the coast, you will do a bit better but the AI will quickly trap you and force an auto-resolve that you won't win.
This is a longshot and probably not possible in the current game but it should be considered for future TW titles: there needs to be an option for "violence without DOW" meaning that you can be in a state of "half-war" with other factions. You raid them but they're unlikely to prioritize you. Once you actually conquer territory from them, or fight one of their armies, then the game is on.
If you take a walled settelement but don't occupy it, you should be able to replenish a good deal of your troops. This is fairly historical for the period. Taking on troops for the enemy army happened quite a lot, and there's no reason why this should be limited to field battle victories.