- Estimation ('market sizing') is a standard part of management consulting company case interviews . The reason is because clients will be throwing you questions and one part of the job is to look smart and give reasonable answers on the fly, without going to a computer or grabbing a calculator first.
- Physicists call them Fermi problems 
- Microsoft (in)famously asked 'How many ping pong balls fit into a 747?' as a brain teaser . This was common enough that someone wrote a book about these brain teasers .
- Fast mental math is a standard part of many trader interviews, since you'll be making split-second decisions under pressure 
One technique is converting everything into log10 first, e.g. 3 billion is about 3 * 10^9 ~ 10^9.5, then you're just adding / subtracting exponents to multiply / divide. Another way is to always round inputs to 'easy' numbers (2, 3, 5), and calculate them separately from exponents.
A few minutes with a napkin can easily save several hours doing something that can't possibly be worthwhile