The phrase is metaphorical; it doesn't mean an actual park walk.

Not Literal

Best used in casual or informal situations, not in formal writing.

Non-serious Contexts

It might imply the task is easy because of one's abilities, not generally.

Emphasizes the enjoyable nature of an activity, not just simplicity.

What's a walk in the park for one person may not be for another.

It refers to psychological ease, not necessarily a fun or literally enjoyable activity.