Can someone tell me the purpose of this multi-tool?
That is a fishing/survival tool.  To use most of the tools, you snap the needed tool (such as a hook) off the slab.  The individual tools do not reattach.
As you noticed, most of the tools are for fishing, but some are more general tools, such as a saw.
Whether or not its appropriate for your 14 year old nephew, depends largely on his maturity and your best judgement.  Personally, I think it's fine for the typical 14 year old, especially if they like to fish.
As a side-note, unique tools like this can be great for arts and crafts.  For example, some of those tools can be used to create wonderful patterns in clay or can be used with paints similar to how you would use a palette knife.  Obviously, not for little kids due to the small metal parts, but for adults, these types of unique metal shapes can lead to wonderfully unique artistic expressions.