If a tool just prints the answer, the student copies it and learns nothing — and the next sum is just as hard. So for graded work, doubt-solving gives you a guided walk-through: the method, the step you were missing, and a similar problem to try on your own straight after.
It tolerates messy input. Photograph a sum in your own handwriting, on a torn or stained worksheet, and it does its best to read it. Snap a whole worksheet and it splits the page and queues each question.
Every doubt you ask is saved into a doubt-history notebook, linked to the chapter. Ask something similar later and it points you back to the last time — so you can see your own pattern.