I think this is a question for which most people have an answer. A quick Internet search will show that some people think it's anti-social, shameful, and somehow destroying the fabric of society. Let's put those people into the misguided bucket for now. The rest of your search results should've yielded a variety of blogs and forum posts ranging from curiosity to advocacy of the solo gaming experience. These people all have their reasons for playing a multiplayer game by themselves. I'm certain you have your own reasons as well.
I can't answer the question posed in the subject of this post, not in a universal way. All I can answer is why I personally play solo, and what I choose to play solo. I think that makes a solid topic for my first post on this blog. Let's see where this goes.
To being with, the games I play solo are table top miniature games and role playing games (RPGs). The miniatures games probably didn't raise any eyebrows. But I'm sure the RPG game play is already coming under suspicion. I've been doing solo miniatures games on and off in some form for years. However, the RPG solo games are new territory for me. In fact, the solo RPG experience is the genesis for this blog. But I'll get to that in a moment.
Table top miniature games are probably the easiest games to play solo. At the most elemental level, you line up opposing armies and run them at each other. The game mechanics take care of just about everything else. Back in the late 1990s when I started assembling the starter box content of Games Workshop's Warhammer Fantasy Battles, this was how I played. My Empire army was on one side and my Orc army was on the other. I rolled dice until one side collapsed, usually the Orcs. This was not very fulfilling, to say the least.
I now actively play many different miniatures games with a great group of friends. However, I still do a lot of solo miniatures gaming. Not everyone wants to play the game I want to play. Not everyone has the time to play when I'm free. Additionally, solo games allow for events or scenarios not strictly covered in the rules. I don't get that out of regular games with friends. I'm not even sure I want that, to be honest. My solo miniatures gaming has evolved from running Orcs to their deaths to trying to evolve a story from game to game. This is a theme I hope to cover in this blog.
Now to consider the RPG. I've been an RPG gamer all my life. I found Dungeons and Dragons back in 1979 and became an addict. I played multiple times a week, straight until the day I graduated college. After that, I played White Wolf's World of Darkness RPGs up until my son was born in the late 90's. But probably unlike a lot of other people, I mostly played with a single DM (also my best friend) who filled out the party with an NPC or two. When I relocated across country, we still kept our game sessions going via chat rooms or even the telephone! (That got costly, let me tell you. It was the days before unlimited long distance.)
After my son was born, I went on a 14 year hiatus from RPGs. This hasn't really been my preferred choice. I still wanted to play RPGs. For years I refused to look at the cool new books, read the blogs, look at the pretty pictures, etc. However, quite by accident, I started to find people on the Internet who play RPGs as solo games. I was amazed and immediately interested. I gave it a shot, and found it can be done. Now I feel myself coming back to my roots.
A few paragraphs ago I mentioned solo RPG game play as the genesis for this blog. I want to play my games, but I also want to share them with someone. I don't care if I have an audience of one person. I enjoy creating the story, and want to have a venue to share it. In many ways it's like painting. I can create a painting and put it in my closet. It was fun to do, but if it was any good wouldn't it be better to hang it where someone might see it?
This community of solo gamers seems fairly small to me, especially solo RPGers. So more sharing of information and ideas can't be far off from a good thing. I'll be putting up both my miniature and RPG solo game reports and techniques I've used to realize a good solo experience. In many ways, you will become my gaming group. Welcome to the table!