It's about time this issue was addressed : for teachers, a very frustrating question (for which a common and very valid answer is 'in your test/exam'); for students, a very relevant and valid question. Hopefully, the following will satisfy both.
Just because one person is not interested in a specific area of maths (or indeed all of maths !), that does not mean that it shouldn't be taught (because others may be interested). Many aspects of maths are abstract and technical and most people will not use them directly, but many everyday technologies depend on theories of physics, which in turn rely heavily on maths. If readers do not understand any details below, they should research such details further (that's right, it takes effort to understand things...).
When very large or very small numbers are written out in full, they use up a lot of space (and ink/lead). Scientists use such numbers a lot, so a convenient way of writing such numbers is using scientific notation. However, more use of this could be made in everyday life to help people understand how to compare numbers. For example, a billionaire is a thousand times richer than a millionaire, and a trillionaire is a thousand times richer than a billionaire; so a trillionaire is 1 000 x 1 000 = 1 million times richer than a millionaire.