She makes the request because she feels that she is falling in love with Boris.
After his departure, Varvara takes charge of her fate and persuades her to indulge her affection and to see Boris.

Katerina eventually yields to Varvara's representations.

A half-mad old lady, who wanders about attended by a couple of lackeys, has previously frightened the sensitive Katerina (who was reared amid family affection, and cannot understand or endure the tyranny of her mother-in-law) by vague predictions and threats of hell; and when a thunderstorm suddenly breaks over the assembled family, after her husband's return, and the weird old lady again makes her appearance, Katerina is fairly crazed.

She thinks the terrible punishment for her wayward affections has arrived; she confesses to her husband and mother-in-law that she loves Boris.

Spurned by the latter--though the husband is not inclined to attach overmuch importance to what she says, in her startled condition--she rushes off and drowns herself.