to buy stuff, you can use
1. your paypal fund (which you can topup using your debit card
2. use debit card (actually you are actually topup your paypal in the process)
3. both

say you want to buy 100usd stuff
if u have 300 usd in your paypal account, method 1 will be used

if u have 0 usd in your account, method 2 will be used (100 usd will be charged to your debit card)

for method 3, if you got less (like 40usd) in your account, paypal will use that 40usd, plus 60 usd from your debit card

*you can fund your paypal account using your debit card so that you can use method 1. but what's the big deal, it is still the same thing. you need to pay using your debit card. 

why people want to withdraw their paypal? mamak dont use paypal.