So Arthur's job is to keep bewitched items from the hands of muggles and he participates on raids. What exactly is a raid? Do members of the ministry come to random wizard homes to check if they have any bewitched items and arrest them even if that bewitched item is their property?
As we see in CS (and other books), the reason for the raid is very much like that one in raids by Law Enforcement in Muggle world:
Tip-offs (e.g. Harry telling Arthur about Malfoys')
Known law-breakers who did that sort of thing before, like Dung:
“What a night,” he mumbled, groping for the teapot as they all sat down around him. “Nine raids. Nine! And old Mundungus Fletcher tried to put a hex on me when I had my back turned …”
As DVK pointed out in his answer to another question, another of one of Arthur Weasley's job duties is writing or overseeing laws regarding, at the very least, the enchanting of Muggle artifacts.
The relevant provide excerpt:
“Yes, Arthur, cars,” said Mrs. Weasley, her eyes flashing. “Imagine a wizard buying a rusty old car and telling his wife all he wanted to do with it was take it apart to see how it worked, while really he was enchanting it to make it fly.” Mr. Weasley blinked. “Well, dear, I think you’ll find that he would be quite within the law to do that, even if — er — he maybe would have done better to, um, tell his wife the truth… There’s a loophole in the law, you’ll find… As long as he wasn’t intending to fly the car, the fact that the car could fly wouldn’t —”
“Arthur Weasley, you made sure there was a loophole when you wrote that law!” shouted Mrs. Weasley. “Just so you could carry on tinkering with all that Muggle rubbish in your shed! And for your information, Harry arrived this morning in the car you weren’t intending to fly!”
This would be in addition to the raid/inspection duties outline in DVK's answer to this question.
The nature of the raids, then, would be to enforce laws such as those Arthur writes.