By functional training do you mean the use of bodyweight and small equipment training? If this is the case I believe the reason it is so popular is because most people with a gym membership are not educated on beginner, foundation exercises. The small equipment and body exercises used in functional training strategically target specific muscle groups to improve strength, flexibility, and endurance to prepare for more efficient cardiovascular and resistance training. This prevents injury and invigorates the nervous system so the body works as one unit for more effective results. I believe functional training can be beneficial in any program, but some may require it more than others depending on their goals, genetics, skill level etc.