Well, that's always a first question for any gaming project. Why?
Part of the fun is that it is a bit "darker" than other typical ancient / medieval projects in terms of knowledge, facts and information - leaves a bit more breathing room for imagination. Yet, new artifacts, new research, new archaeology, and most importantly - new figures and games are being published all the time.
My initial foray into the period was 25mm using Terry Gore's great traditional wargame, "Medieval Warfare" which at the time checked a lot of boxes for me. Plus the guys that play it and 'con it are a great bunch. Terry himself has passed to that great game table in the sky, but a very positive legacy lives on. For that set of rules I purchased hundreds of Feudal English, Anglo-Dane and Welsh figures. I ran them at many MW tournies including Cold Wars, Historicon and Fall-In! in Pennsylvania. The Feudal English were often winners, and the Welsh so dangerous that they started to change the rules to try and stop them!
Part of the fun of the period is that it is such a free-for-all in post "Fall of the West" Rome. The Barbarians are on the move, and they want your land and your loot! Also, they're a colorful bunch with a lot of expertise in small-scale fighting. In the large battles against organized opponents like East and West Roman field armies, they had poor results unless the Romans made a major fubar. But in raids and encroachment, they were experts - or at least the ones who survived were experts. This makes them less fun to play in big-battle games like WRG7th / Warrior, Field of Glory, DBx, etc. But in characterful small-scale games, they're perfect to play.
Lots of medieval war and rules favor cavalry - often quite unrealistically. It's sometimes more fun to play an all-infantry encounter, especially in a skirmish. The ideal set of rules for fun skirmish games at present is SAGA, far as I can tell. It has plenty of space for both solid tacticians and those who comprehend the rules of their faction on the battleboard. And they don't overpower the cavalry armies!
Honestly, my interest in the period was flagging until I started to investigate more of the writing around the period, especially the real sagas and better fiction writers like Sutcliff. Then I stumbled upon Sash and Saber's extensive Dark Ages line of Vikings and Saxons [Anglo-Danes]. After that I was just trying to find an excuse to buy them. Eventually I sold off enough other game projects that I felt like I could buy them without remorse. I enjoyed choosing the poses and prepping the first 28 for paint. Then real life intervened for a long while and now with a bunch of SAGA games under my belt, and no 25mm Vikings ready for the table, I'm ready to push this project forward.
While the details of the first purchase and selections are passed, I'll post a bit about how to get started in a reasonable fashion with the 40mm, especially with SAGA in mind.