Here’s why this configuration might exist and how it might function:

• Security from the Inside: Having a key lock on the interior handle can provide an extra layer of security, preventing unauthorized opening of the window even if someone were to tamper with the exterior handle or the window mechanism itself. This could be particularly useful for ground-floor windows or windows accessible from balconies.
• Child Safety: A key lock on the interior handle can also act as a child safety feature, preventing young children from opening the window and potentially falling out.
• Ventilation with Security: You might be able to slightly open and then lock the window with the key, allowing for ventilation while still maintaining a degree of security.
• Handle Operation: Both the interior and exterior handles would likely operate the window’s locking mechanism (such as espagnolette rods or shoot bolts) when the key is not engaged. The key would then add an extra layer of locking.

It’s less common to see a key cylinder only on the interior handle of a window. Typically, if a window has a keyed lock, it’s accessible from the exterior for security against intruders. However, the configuration you described serves specific security and safety purposes from the inside.