Normally in most offline games you would be forced to play with non player characters also known as NPCs. When you have the ability to match yourself up again or partner up with other real players it can really improve the amount of enjoyment you receive in return for your time spent in these games. Nobody likes receiving nothing in return for their time spent doing something so why would you not maximize the enjoyment you get out of playing games.
Comparing yourself to other players in an online environment can give you a goal to strive to achieve. When you see someone else playing who is really good it makes you want to try to be just as good or even better than that player. Having these strong motivators can really improve how good you are as a player, and when you are a better player you might even end up enjoying yourself more. It may not always be competitive when you are wanting to match up to other players, you may just be wanting to meet the standard.
The social interaction you get through online gaming is really unique. It is no secret that people enjoy talking to other people. This same factor is also why Chat Rooms are becoming more and more popular every day. Chat rooms and video games both share the factor where you can communicate over the internet with other people about a variety of things, there is even chat rooms about video games. Everyone gets lonely from time to time, online games give you a way to have fun and meet new people at the same time.
Most online games have your progress stored server side. I can not remember how many times I have played games and then lost progress due to me losing the save file for whatever reason. The convenience of modern online video games is the fact that games will store your character or profile on their servers meaning that they will keep track of it for you and leave you to just focus on playing the game. When you have nothing to worry about but playing the game and having fun it can really lead to you having an all around amazing gaming experience.