Being able to do a quick back of the envelope calculation allows you to get an idea of what sort of answer you should be aiming for in a problem. Physics is all about the real world so we are always aiming for answers that make sense.
We looked at the prefixes we use for big and small numbers:
We then started playing with Fermi questions. There are lots of resources to explore on this:
Some take it to university level:
And some that answer the questions we have all been thinking (all the physics in this is in the year 12 course):