Bitcoin debit cards can be seen as a bridge from Bitcoin to the traditional financial system. They make it possible to spend your bitcoin anywhere credit cards are accepted.
While Bitcoin enables people to transact peer-to-peer, this feature can only be taken advantage of within the network itself. If your local furniture store doesn't accept bitcoin, you'll have to pay via another method! Further, Bitcoin (at least in its current state on 'layer one') isn't particularly useful as a medium of exchange for small-value items like your morning coffee or daily groceries. Why?
(1) Transaction fees are often significantly higher than with status quo payment networks like Visa and Mastercard.
(2) Transaction times are significantly longer than with status quo payment networks. Depending on the fees paid and the current level of network congestion, it takes anywhere from a few minutes to an hour for most Bitcoin transactions to confirm 'on chain' - and only when a transaction is confirmed on chain, meaning it's included in a least one block, can final settlement be considered to have occurred.
For these reasons, Bitcoin's use as a medium of exchange is currently restricted to higher value items where transaction times and costs are less consequential - like buying a car, boat, or house.
Note that so-called 'layer two' solutions like the Bitcoin Lightning Network solve the above-described challenges by enabling 'off chain' transactions. This is similar to how the Visa/Mastercard network functions in that millions of small transactions can go through quickly while final settlement occurs in large batches at a later stage. Unfortunately, Lightning Network is even less widely deployed than the standard 'layer one' Bitcoin network, meaning the places you can use it to actually buy things are few and far between. Therefore, for the time being at least, Bitcoin debit cards help make Bitcoin useful as a medium of exchange while layer two solutions are still in their infancy.
Bitcoin debit cards are like prepaid credit cards. You can use them to make payments in person or online exactly like you would with any other credit card. You either enter your card details or swipe your card to make a payment, and merchants receive cash in their local currency. Many Bitcoin debit cards also allow you to take out cash from ATMs.
The two methods for funding Bitcoin debit cards are:
Depending on the provider, your Bitcoin debit card may or may not have an annual fee. You may also be charged a small fee each time you convert your Bitcoin to cash.
With virtual Bitcoin debit cards, you're restricted to making purchases online. Physical Bitcoin debit cards can be used both online and in person at any merchant that accepts the credit card type (eg. Visa or Mastercard).
Since Bitcoin debit cards are offered by registered businesses, you'll need to provide identity documents in order to be approved. This isn't a credit check, but rather a requirement for compliance with anti-money laundering regulations. Each Bitcoin debit card provider can only offer its card in the specific regions it has obtained the required license.