Tosfos answers that Serach was the one to whom the secret of the Ge'ulah was handed down; she was the one who knew exactly what the redeemer was destined to say when he came to redeem the people from slavery. Hence it was to her that the people turned when Moshe and Aharon appeared to take the people out of Egypt.
And therefore she was the one to inform the people of Moshe and Aharon's authenticity once they used the words "pokod yifkod eschem". Moreover, Tosfos explains, following the large contingent from Efrayim who left Egypt thirty years before the due date of redemption, the people no longer trusted Yosef's descendents. So they consulted Serach. That is why Moshe Rabeinu consulted her too.
The Gemara then goes on to describe how Moshe stood on the bank of the River Nile at the spot that Serach had located. There he announced that the time had arrived to take Yisrael out of Egypt, and that he had come to fulfill the promise that Yisrael had made to him (Yosef) to take his coffin with them when they left, but that if he failed to reveal his whereabouts, they would be absolved from the promise. Immediately, Yosef's coffin floated to the surface and Moshe retrieved it.
A number of Medrashim have been written about Serach. According to one medrash, Serach was not Asher's daughter, but his stepdaughter. She was three years old when Asher married her mother, and she was brought up in the house of Yaacov, whose affection she won by her remarkable piety and virtue.
The most well-known of the Medrashim tells of how she was the first to inform Yaacov that his son Yoseph was still alive. Fearing that the news will be too much of a shock for the old man, however, she informs Yaacov by playing a harp for him, gently mixing in the words that Yoseph is “alive and the ruler of all Egypt.” In return, Yaacov blesses her, saying “May you live forever and never die.” According to this medrash, Serach was eventually permitted to enter Heaven alive, something achieved only by Enoch and Eliyahu.
According to the Medrash, Serach was "the wise woman" who caused the death of Sheba ben Bichri, which ended a rebellion against Dovid. According to another legend she lived until the tribe of Asher was exiled by Shalmaneser V, went with them into exile, and died there, being nearly 1000 years old. According to the legend, her grave is located in Linjan, a small village in modern Iran, about 30 km southeast of Isfahan. The site consists of a small synagogue and a huge cemetery which is probably 2000 years old.
