Not sure about other countries, but in the UK you need both - a bank account and a card connected to the PayPal account. As PayPal puts it - it's because the card is your backup payment method if the bank payment (also called "Instant Bank Transfer") fails.
There's some more information here: http://forums.moneysavingexpert.com/showthread.php?t=4055559
not true, im from uk had pp account 8 yrs
i did have a cc on my account until 2008 but after 2 hacks i removed it
no problem with pp there, just means funds only go 1 way, pp>bank
not the other way