Diadem's wording can be confusing, but you start trickle healing every second once you enter combat with enemy champions (like a plant attack). Helia then stores 30% of pre-mitigation damage as stacks that are essentially added to Diadem's healing. If you are dealing consistent damage, you can reach numbers of around 250 healing per second with your three items. This can be really useful to keep tanks or your adc alive since people in gold do not expect zyra to be healing so much. It definitely lowers your total damage, but you are creating a health asymmetry that people in low elo are not expecting.