It depends on your age, health level, and dedication to training.
A heart rate monitor is a great way to find a numerical level of “effort” you are engaging in. If your heart rate is 160+, you know you are really engaging in a lot of effort, whereas if your heart rate is 90 you know you are in the ‘warming up’ zone of effort.
Addressing your core question, I highly doubt that using a heart rate monitor to exercise at the ideal zone would have hardly much effect compared to simply adding another day of exercise to your routine. We’re talking about using the heart rate monitor to squeeze out an extra few percentage points in terms of efficiency, this is easily surpassed by simply exercising more. If you don’t have time or desire to exercise more, then perhaps the few percentage points is of value to you to justify the equipment cost.
If the trainers claim they “help a lot”, call them out on it, ask them how. The burden is on THEM to prove to you how much they help, and if the proof is anything beyond scientific experiments I would consider it complete rubbish. As a generalization I would say that a heart rate monitor “helps a very little bit” if it helps at all. If you purchase it expecting it to have a significant effect, I fear you would be disappointed with the little if any results.