i think estimating is one of the most important skills in maths (well, arithmetic/common sense) that is rarely taught in schools. for example, estimating the cost of your supermarket trolley, estimating how much wallpaper you will need to paper a room, etc.

now, obviously you want to get these things accurate, but the estimate tells you when you got the calculation wrong in your measurements, spreadsheet or calculator.

i was guilty of this when i over-valued my late-dad's book collection in a python program. i sort of knew it was wrong but what i didn't realise he had changed his data format halfway the dataset.