Heart rate monitors (chest strap or wrist) reveal actual training intensity more accurately than subjective perception. Most users find they train at different intensities than they thought — usually less hard in 'hard' sessions and harder in 'easy' sessions.
What this enables
Genuine Zone 2 training (60-70% max HR — most aerobic base work). True high-intensity intervals at proper zones. Better recovery tracking (resting HR over time).
What to look for in a monitor
Chest strap most accurate (Polar H10, Wahoo). Wrist-based less accurate but more convenient (Apple Watch, Garmin). Either works for general training; chest strap matters for serious intervals.
If you've been training without monitoring HR, one month with monitor often reveals you're not training where you think you are. Adjustments produce better outcomes.