Retailers get charged about 20 cents per 100$ for debit transactions and $1.48 per 100$ for credit. If you say credit with your debit card you are giving your bank that larger cut and getting none of the protections associated with a credit card. Except you probably don't have to enter your pin number which may be important to you.
http://redtape.msnbc.com/2007/09/paper-or-plasti.html
Maybe it's different in your country, but in the UK if you pay on credit card and the supplier fails to deliver your credit card company has to refund you, it's then upto the credit card company to get their money back from the supplier.
In the case of debit cards you don't have any of that protection, so if you want to get your money back you have to get it back from the supplier (which if they've gone bankrupt/disappeared is near impossible for an individual).
Yeah, that works, but oftentimes the protection isn't as good. For example, with many credit cards at this point you are liable for $0 worth of fraud, but with the debit as credit you are liable for the first $50.
Either way deducts money from your checking account in the same method, but using it as debit requires your PIN and using it as credit requires your signature.
