What is the "hierarchical" about in Hierarchical Deterministic Wallets?
Hierarchy allows interesting use-cases.

You could have a master company key m/0' and give out m/0'/0/0 to company-branch A and m/0'/0/1 to company branch B, etc.

You could then allow audits by handing out the extended public keys (xpub) of m/0'/0 to some trust organization (or just hand out m/0'/0/0 if you want someone to audit branch A only).
Private key at m/0' can reconstruct all funds from all branches.

On top of that, online-shop of branch A could use the xpub of m/0'/0/0/0 to received funds while not revealing the wallet structure of m/0'/0/0, etc.

Another interesting use-case would be to map domain-names or other string-based- identity-systems to the hierarchy. Example: Github.com could result in m/71'/105'/116'/104'/117'/98'/... (very unoptimized way for demonstration purpose).