Understanding recursion tree for withdrawal formula
Your recurrence expands into a recurrence tree in the following way. Each node has a label. If the label $n$ is $a$ or smaller, then the node is a leaf. Otherwise, the node has two children, one labeled $n-a$, the other one labeled $a$.

Suppose that you create this tree starting with a root labeled $n$. The root has two children: one is labeled $n-a$, and the other one, labeled $a$, is a leaf. The node labeled $n-a$ has two children, one labeled $n-2a$, and the other one is a leaf. And so on.

It takes roughly $n/a$ steps until this process terminates (since we start with $n$, have to get to $a$ or less, and drop by $a$ each time). Therefore the height of the tree is roughly $n/a$.