Trail of Bits gives this excellent explanation of
These competitions distill major disciplines of professional
computer security work into short, objectively measurable
exercises. The focus areas that CTF competitions tend to measure
are vulnerability discovery, exploit creation, toolkit creation,
and operational tradecraft.
Basically, CTF competitions are cybersecurity competitions that allow students an opportunity to study and apply the many diverse topics related to information security. Many of these subjects are not covered in the course of obtaining a Computer Science degree, so CTF competitions provide a place to learn and hone these specific skills.