The generally-accepted reason for the guarantee is to counter the disgrace that she had to undergo as a Sotah. Perhaps one can add to this what Chazal learn from the very same Amud (from the episode with Eli ha'Kohen and Chanah) that someone who suspects an innocent fellow-Jew of sinning, is obligated to bless him. Consequently, the Torah, which suspected the woman of having committed adultery, when in fact, she did not, follows this up with a blessing.

I did not find the explanation that you wrote. In fact, I find it a little difficult, because how do we know that this is what happened.

And what's more, I have a proof from Chanah that it is not the reason. Because if it was, how could Chanah expect Hash-m to reward her with children, bearing in mind that she was not in the least interested in whichever man she intended to seclude herself with (nor do we know that he would have been in the least interested in her), in which case there was no reason to think that she would need to resist his advances.

Eliezer Chrysler