.io games became popular with Agar.io game release. It had simple gameplay and graphics comparing with modern PC and even smartphone games. Nevertheless, this characteristics + many online players made this game super popular. It has a lot of clones and modifications that continue to come up every day. Next there was another great game Slither.io. After those a new .io games trend began. We have the most complete list of io games and keep it updated.
— simple and obvious gameplay
— multiplayer mode and many online players
— the games are easy to begin, since the games are usually have continuous gameplay, new players join the game where others may already have been playing for hours; io games are well balanced so newbie can compete with advanced players
— team play
Unlike mane "freemium" games io games are absolutely free, complete gameplay is available without extra payments. There're some ads on splash screen, but they don't bother much. Sometimes game may require registration, but that's an exception. Some games are available at Apple Store or Google Play, but PC with mouse and keyboard may be an advantage, especially in strategies, like Moomoo io. Moreover you can't download io game to you computer, because it won\t be an io game anymore)
You might consider io game running on any device since it's browser game, but you'll be mistaken. Since game runs inside browser it heavily uses devices resources. So on an old computers the games might be laggy. To avoid those you need not very old computer and a high-speed network connection. If the game still lags, try exit full screen mode. Also in some games you can reduce graphics quality.