You’re right that if you know the exact range of values that a column will have then you would know the exact minimum. But, if there is variation between different batches of data, then it may make more sense to expect the minimum of any given batch to fall in a range. For example, you might expect a minimum transaction value between 0 and 1 dollars if you may have variation inside a day, but you want to ensure there are always small transactions in your data.