Ok, here's how it works:
You take testosterone, and your body freaks out a little. It detects a sudden increase in androgens, and two things happen; one, it turns off the signal to your testicles to make more testosterone, and two, the aromatase enzymes in your body go to work - converting some of that test into estradiol.
The body is very smart in many ways, but very dumb in others. It will try its hardest to try to maintain a balance between testosterone and estradiol, even if this causes problems. As a cycle vastly increases your testosterone, you don't notice much with the exception of your testicles shrinking/hugging your body - they're pretty much put in timeout. However, those pesky enzymes start working overtime to meet this new demand to keep a precious ratio of T:E, this is even worse if you happen to have a significant amount of visceral fat, as that gives your body additional workers for the task at hand.
This is where the aromatase inhibitors come in to save the day. What they do is essentially occupy the aromatase (some permanently like aromasin), so it cannot continue to escalate your estradiol. On average, the proper dose will keep this in check from 60-97% of the total output.
This is good and bad.
It's great as you don't want to see that estradiol climb unchecked as you may grow breast tissue, piss off your prostate, hold water like a camel, possibly get some blood clotting, and likely join a knitting circle with your new found feminine side. Not to mention, your penis may go on strike.
It's terrible in that we NEED some estradiol to function. Bone density can suffer, your skin and nails may become brittle, you'll have zero energy, and your penis (being such a cooperative fellow) may go on strike. This of course being if you overdo it.
This process is always going on while cycling/using exogenous androgens, and requires constant upkeep. This is why blood tests are preached so heavily here. What works for Johnny may not work for Billy, but it does for Bobby. Due to this scenario, guidelines (more like rules of thumb) are put into stickies as dose suggestions - but a blood test lets you KNOW for certain what dose works for YOU.
TL;DR: A blood test keeps your penis from going on strike and having to find a bikini that matches your eyes. Too much or too little AI are equally bad in the grand scheme of things.
My .02c