Rounding :
Most of the time you don't need too much details on a number. Those might look pretty and informative on presentation but hard to grok.
"The distance is 2834 km" -- might be accurate but hard to comprehend.
"The distance is 3000 km" -- you can comprehend it easily. Even if it's not 100% correct.
And 2834 is approx equal to 3000.
Therefore the rule is to round all numbers to its first digit only. You can now better understand the quantity, and work with those numbers further.