Much of what paralegals do (researching legal questions and writing legal documents, for example) is the same work that attorneys do. So why not become an attorney instead -- especially since attorneys make more money?
Some people choose the paralegal route because they don't want to go through three years of law school in addition to receiving a bachelor's degree. Others want a meaningful way of participating in the legal process without the demanding schedules that attorneys face.
Of course, there's no reason you can't do both. Some people start out by getting paralegal training and working as a paralegal for a time, and then go to law school.