0.5 is the same as one-half. If you are walking home from school and you are one-half of the distance to your home when it starts to rain, would you keep going home or turn around and head back to school?
Do you think it would be reasonable to give everytime all the numbers in pi for example?
One other thing... when you take a reading, you always take it to the nearest reading you can measure. If the exact length of something was 1.0652 cm, you will only be able to measure a length of 1 cm with a common ruler.
Rounding off is a means to simplify numbers and remove unnecessary details. This eases work. Where some accuracy is required, then you don't round off.