Timur did not seek to conquer lands away from the southern branch of the Great Silk Road – he either plundered them (like India), or devastated them, like the Golden Horde, which controlled the northern branch of the Great Silk Road. Russia, according to Timur, was too poor to plunder, and hostile to the Horde, and therefore useful to Timur.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.