It's very common that teeth become discolored after root canal treatment because the dead tissue's pigment penetrates into tooth structure. Aesthetically it doesn't look nice especially for front teeth.
Veneers and crowns can cover the discoloration from outside. But they are not only expensive, more importantly, they also damage teeth by removing tooth structure. Internal bleaching whitens the tooth from inside using bleaching agent. Compared to porcelain veneers and crowns, it is very affordable and conservative.