I make all the purchases that would ordinarily be paid in full with the card (utility bills, gasoline, groceries, etc...) so I can track my spending habits and trends. It's great for sticking to a budget. I enter all the info to an Excel sheet, and I can make graphs to gauge how I'm doing. It's easier to keep track of things, but I pay the bill in full every month.
Be very careful with them however, credit card companies are notorious for their deceptive advertising. The stated rate might seem reasonable, but it may be temporary, and cash advances, late payments, etc. usually carry a MUCH higher rate. Some over 20% which is horrendous.
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O Wonder! How many goodly creatures there are here! How beauteous mankind is!