A mentor gives context that textbooks rarely know where to put.

Formal learning tends to give you the correct content and leave out where it fits, when it matters, and which parts people actually use. That context is precisely what someone with experience holds, and it is rarely written anywhere because it feels too obvious to record. It is often the difference between knowing a subject and being able to work in it.

When you help someone, tell them what actually matters in practice and what is safe to hold lightly. That ordering is the thing they cannot look up.