As has been mentioned, wallets can conceal razor blades and other possible weapons. The old method of simply putting the wallet on a small basket, bypassing the metal detector, is obviously less secure.

The jacket is the same idea. With a jacket on, it's hard to do a visual search or a patdown. That's why they prefer to simply X-ray it.

Personally, I put all my change, gum packet [the tinfoil sets off metal detectors], wallet, and belt into my jacket pockets and just pass the whole jacket through. You can't walk off with my jacket without me noticing.