Why, when using mechanisms for lifting loads and overcoming any resistance, is useful work not equal to the full?

In practice, full work is always somewhat more useful (if we talk about the mechanisms). And this happens because part of the work is spent on overcoming the frictional force in the mechanism and on moving its individual blocks.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.