Fundamental: You make yourself an easier target an easier target for enemies.

When you use your Blood Offering feature you can instead choose to gain a stacking penalty to your AC instead of reducing your Hit Points. Taking a -1 penalty to your AC is equivalent to spending 10 Hit Points with Blood Offering. You can use Strength in Vulnerability interchangeably with spending Hit Points with Blood Offering. You can reduce your AC to as low as half your total AC (so if your AC was 30, the lowest it could be reduced is 15). This AC penalty lasts 1 minute. Subsequent uses do not refresh the duration