Question

Why would you round to nearest 100 rather than nearest 10?

Answer 1

when you're dealing with quantities where getting the nearest 100 is good enough. That's the whole point of rounding. If you're estimating the size of a concert crowd, who cares whether there are 1623 or 1630 people there? You might want to say that there are around 1600 people present. Or maybe even "a couple thousand" to give an idea of the number.