They are a way of giving up any semblance of security, in return for saving a few seconds entering you PIN.
If you lose it, anyone who finds it can spend as many £20s as they like, until you cancel your card, and then have to wait for a new one.
Or, as has been happening in bars in London (and other places - not here, where the bar staff are all too honest) ...
You buy a couple of rounds in the bar. The bar staff then ring through odd, small amounts, say £12.65 or £16.20 ... squeeze past you in the crowded bar as they go to collect glasses ... brush the machine against your pocket ... and your card authorises the payment. Then the bar staff take that amount out of the till.
A few days later, when you see your statement, you're hard pressed to prove that the £14.90 was yours but the £12.65 wasn't, and the £15.85 was yours but the £16.20 wasn't.
Best advice ... if you really must have a contactless card ...
1. keep it in a locked case in your bag, with a combination lock, in case you lose it, and
2. make sure the case is lead lined so you don't get caught by the "brush against" scam.
or ...
3. just have a card which at least requires a PIN number.