Round large numbers when you only need an estimate  not the exact number. Rounding numbers makes them simpler and easier. To round to the nearest million, look at the number in the hundred thousands place and then round up or down to the nearest million.
Understanding Place Value

First, find the millions place. This number is the seventh number to the left of the decimal point. In the number  7,654,321  the seven is in the millions place. You will round to this number.
Next, find the hundred thousands place. This number is the sixth number to the left of the decimal place, or the first number to the right of the millions place. In the example number  7,654,321  the six is in the hundred thousands place.
Round Up or Down

Look at the value of the number in the hundred thousands place. The rule of thumb is that if the number is five or greater, then round up. If the number is four or less, then round down.
Since the number in the hundred thousands place is six, you round up. This will increase the millions place by one to 8,000,000. Notice that the rest of the numbers become nice round numbers, or zeros.
If the original number is 7,354,221, then the number in the hundred thousands place  three, is less than five. In this instance, you round down  which means to leave the number in the millions place where it is. This example rounds to 7,000,000.
References
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