Why would I want to know my heart rate? Having played a musical instrument, I can check accurately enough just by putting my hand on my chest if I really want to know for some reason.
Well, for those of us who take our running or other exercise seriously, there are 4 specific heart rate zones ranging from couch potato to imminent heart explosion. OK, technically it's minimal - about 50% of heart max, weight loss, about 65%, aerobic, about 80%, and anaerobic, 90%+.
We need to know what zone we are in to know if we aren't working hard enough or too much depending on what we are trying to accomplish. This is especially important when targeting aerobic or anaerobic workouts because they do different things to the body. So that's why a heart rate monitor is a great tool. Right now a heart strap is the most accurate method, but it can be uncomfortable and a PITA. So just popping in earbuds would be a huge improvement IF its as accurate as a strap.