You can defend yourself to a degree from physical phenomina, and completely from magical attacks. But the point isn't offense or defense from or at things. The point of using magic is to get in touch with and work with nature for the ultimate goal of bettering yourself and the world.
you can use magick for whatever you desire as long as you take responsibility for your actions. the point of magick is to use that which was given to us and is our natural birthright. it is also their to make our lives much easier by getting what we want. Money, Love, good health, revenge, protection, career, fame and success etc and so much more can be yours. you only have to believe
well you can defend yourself with magick. you can't freeze a bullet, or stop time to avoid an attack, but you can make charms or cast spells to protect yourself, the people around you, and your possessions from harm.
magick is used to create change in ones life. most will got he positive root [such as luck or healing] but you can use it for revenge on people you feel deserve it. it's all morality really. as for magick's uses, it's using natural energy to create change in your life, good or bad. it cannot go against nature, but it can make your life easier.