Police investigations cost hundreds to multi-thousands of dollars depending on the manpower and resources required to bring a case to a close. One tip to crime solvers can eliminate hundreds of man-hours in background checks and field investigations.
Many tips lead directly to the criminal source.
There are several reasons. The first, people just don't want to get involved in the often drawn out court procedures. On the other hand, they don't want the criminal to go undetected either. Others just think their information might be helpful in general. Concerned citizens, ex-lovers, drug dealers, and criminal associates.