One point to note is your HR won't rise if your cadence is low, no matter how hard you pedal. You need to get your cadence up to 95 rpm for your heart to be used efficiently.
I use mine on the turbo as reference for how my body is feeling the effort. What your head and legs say is different to your heart.
So let's say you do a routine of intervals and at each change you record your max and average HR. When you repeat the same session a month later you should see that the HR average and peak have come down, if you have been training!
You also get thermal drift, what I mean is if you pedal at the same rate and load for an hour your heart rate will creep up.
You can expect your base rate to vary with temperature too. Generally the warmer it is the higher it goes up.
Out on the road there are a lot of other variables like wind and temperature so as previously stated power meters are more popular.
You can also use it to restrict your effort. Lets say you are doing a long slow ride you can ease off when your HR goes over a certain level. The reason for doing this is so you get to build your aerobic base and separate it from your other training. A long hard run kills your strength and speed work later that week. As you get fitter your speed will increase at the lower threshold. Maffetone recommends (180 - age) for this, regardless of your fitness or age.
What have used is my (max HR - min HR) to calculate the range my heart can peform. Then break that in to 5 zones. IHMO you only really need about 3, 1 to keep your pace down for base aerobic training, 2 for what you want to sustain for a given period of time or race and the third for pushing your self beyond your aerobic limit into lactate threshold stuff.
HTH
( Alternativley you can wear it in bed and use it to show when yer missus is not trying hard enough
)