Let's take this apart nicely and easily.

Egwene comes into the room, spots a significant herb garden on the balcony and various shrubs and small trees, and in one corner something for carrots and radishes to grow in. Food is certainly not the prime focus of this interior garden.

Egwene, because of her training as apprentice to a Wisdom, has knowledge of herbs and their uses. How much knowledge, we don't know, but she hung around Nynaeve for quite a while and from the quizzes Nynaeve gave her in the first couple of books, she knew her stuff. Follow that with her training with the Wise Ones and that knowledge should be pretty comprehensive.

She is able to, at a glance, recognize over fourteen herbs in the garden. She names two of them, two herbs with known medicinal properties in our world. Basil and thyme are *NOT* spices. Basil has been used for treating asthma and is being researched for antioxidant purposes. Thyme is an antiseptic and was used to prevent infections as a poultice.

So, far from thinking Suana's food had medicinal purposes, Egwene clearly identifies herbs, herbs she knows Nynaeve would use and could use herself. Whether it is a leap of faith or a logical flow, she now realizes that unlike the majority of Yellows she has met over the couple of years she's been in training, here is one that doesn't eschew the stigma of herbal medicine.

Since a Yellow is unlikely to develop an attitude like that to herbs late in life, it isn't too much of a reach to assume Suana came from similar roots to Nynaeve. With the ridicule that the Yellows have shown over Nynaeve's herbs, finding that one of the Sitters shares the reverence for her early roots is a revelation that would, and did, change her own attitude to the Ajah. Don't forget Nynaeve has a biased view of the Yellows, a view she passed onto Egwene in Salidar.

As to why Suana would invite Egwene to the Yellow despite her poor healing skills, I think that has been debated correctly above. Egwene is powerful and Suana wanted added strength to enhance the Yellows' position in the fragmentary hierarchy of the Tower at the time.

My old English teacher would have given you 3/20 if you handed that mess in as an answer to a reading comprehension test.