It isn't always practical to weigh something.
So sometimes an estimate is more appropriate. When estimating the mass of an object, it is useful to have some point of reference.
Do you know how much you weigh in kilograms? Would that be more or less than a domestic cat? How about a tiger? It's easier to estimate the weight of something when you compare it to something else.
You can also consider the weight of a similar but differently sized object if it would be made from the same material. So if I know one bag of flour weighs around $500$500 g, I can estimate that $10$10 bags of flour would weigh $5$5 kg.
What is the most appropriate measure for the mass of a dog?
The container shown has $10$10 kg of sand in it. Estimate the weight of the container when it is full.
The container shown has $4$4 mg of foam in it. Estimate the weight of the container when it is full.
Not to scale
Measure at a level of precision appropriate to the task
Apply right-angled triangles in solving measurement problems