Not only is it a seed planter, but also for helping get fertilizer down to the root system

It's a tool specifically for use in places that have notoriously hard soil - used alot in South America, 'bush' Australia etc. the spikey part basically makes the hole, the green part opens up the soil, and the hole in the side is where you pop the seed down. Once you want to start fertilising, you do it a little away from the plant growth, to get the fertiliser treatment down to the roots.
Silly little bits of information - they come in handy some times
I remember my dad had something very similar when he had his vege garden in Australia when I was a kid