Comment by Aardwolf

And the reason subnormals exist at all is that normally there's an implicit 1-bit (without the implicit 1-bit, there would be multiple representations of the same value, e.g. all values with mantissa 0 would represent 0 regardless of exponent), but you could never represent the value 0 itself if the implicit 1-bit is always there. So for the lowest exponent value, the implicit bit is set to 0, to allow representing 0, but also those other very tiny values since there's still an entire mantissa to give meaning to in addition to value 0.