For me it means shorter more effective training. I can accurately target the effort/load. So on an interval session I can confirm Im at 90-100% or zone 5 effort for the required time. Yes there is lag changes over time but thats why you use a zone.
HRM is for me the most cost effective option when its linked to uploading the data in to the free online training diary. I can then track training and identify performance increases, rest sessions, weight changes speed distance. Also handy for setting goals. I use a Polar HRM.