Cardio Resistance Levels & Target Heart Rate

I am 27 M 6'3",236 lbs. I have recently started with Cardio,mostly exercise bike,treadmill and elliptical. Now i mostly exercise on the bike and the elliptical at resistance level 1. I've read that we should increase the resistance levels for a better workout. I want to know is it really necessary to do so if i stay around my target heart rate (around 150 bpm ) for most of the workout. Even if the resistance is increased won't the motive be the have a higher heart rate and hence a better workout ?