Because it confirms that she's one of the few Lightsiders who can claim a Forsaken scalp. Two in fact ... other than Rand no one else can claim 2, justifying the credit she was given by others in the series.

I don't know that I'd give Perrin credit for setting the circumstances since he didn't do it with any knowledge of what he was doing (i.e. trapping the Black Ajah), in his case it was lucky timing that trapped the Black. However, Mesaana deliberately set the stage for the confrontation with Egwene by deciding to take her to a spot where it was just the two of them in a head to head fight. Egwene defeating Mesaana was all about Egwene having a powerful will. Even if you consider Mesaana one of the lesser Forsaken, it's still impressive to take down one of the Forsaken.