The first map that was added in a fantasy book is Middle Earth, drawn by Tolkien himself. Sometime later, we can see many other works followed suit with fantasy maps added. The map is not necessary and not a must, but it'll help the reader become familiar with the world you built.
Honestly, I do love drawing fantasy map sometime. But after all of the hardship(of researching tectonic plate, the sea level, natural terrain, mountains, trenches, river, and lake), I found that the map of my world isn't that of importance. Or maybe it's just not working for me. And above all, it's all too much of a hassle. Though I'm still doing that ..., drawing maps.
If it could help you bring out the clarity of the story, then yes, it does necessary.
But if it's a world map, while the story only happens in a small single town and the MC didn't even step out of his habitat, then no. <- fyi, I did that in my story some time ago.