قَالَ هِيَ عَصَايَ (He said, “ It is my staff” – 20:18. The simple question which was put to Sayyidna Musa (علیہ السلام) i.e. "What is in your hand?" called for an equally brief answer, such as, "It is a staff'. But he volunteered additional information which was outside the scope of the question put to him. First, he said that the staff belonged to him; second, that it served him many purposes, namely that he often reclined on it, and also beat down leaves from trees for his goats; third, that he put it to many other uses. This long and detailed reply is a perfect combination of extreme love and adoration on the one hand and profound reverence on the other. It is a natural human instinct that when a person finds the object of his adoration to be kind and attentive, he wishes to prolong the conversation in order to get the best advantage. At the same time the dictates of extreme respect require that the conversation should remain within proper limits and not become too lengthy. For this reason he ended his reply with a brief statement وَلِيَ فِيهَا مَآرِبُ أُخْرَىٰ i.e. "And for me it has many other uses", but he did not give any detail of those "other uses". (Ruh and Mazhari)
From this verse Qurtubi has deduced in his Tafsir that when needed, it is permissible, while answering a question, to include matters which are not specifically covered in the question.
Rule
This verse also shows that carrying a staff is a practice followed by the prophets. The Holy Prophet ﷺ also used to carry a staff in his hand and this practice has numerous religious as well as mundane advantages.