There's also the concept of a "Home Game" -- your main game that you play all the time and know inside and out.
Because of the small goals oriented way I play any game, I'm able to switch games pretty easily. As I've mentioned, I'm happiest when I can play on my weekly rotation schedule, playing seven different games throughout the week.
Sometimes one game will take over, and becomes your preferred playscape, as World of Warcraft currently has for me. As I'm working Legion, though, I'm getting twitchy, because I don't care for the areas and stories, and I feel like I'm grinding towards nothing. In Legion crafting is just not fun. It's like the game up through Pandaria is one game, then Draenor changes things up considerably, but all things considered, I love Draenor, but Legion...agh. It's a fight with myself to play through it.
As this WOW adoration and exclusivity is crumbling, I've peeked back into Everquest 2, because it is still my current second favorite world, and Ravalation's post on the character models of LOTRO http://ravalation.blogspot.com/2017/08/lotros-first-character-update-humans.html and Myndariel's new Hunter guide http://myndariel.blogspot.com/2017/08/introducing-hunter-level-1-20-high-elf.html has me updating the LOTRO client as we speak. Getting the Mordor expansion as well, because maybe one of my other characters can get past Mirkwood more easily. Let's bring that Rogue forward, right? It's silly to let one character/class get stuck, and let your whole game get stuck, when sometimes another class can break through a problem area for you. Once you know the way, you can anticipate and compensate for the play style of your other classes. Yay for Alt Armies. (Bonks self on head for letting myself feel stuck here).
Having Many Places To Call Home Means You're Always Home
I do admire on occasion what people who only play one game know about a game, their expertise is great when you have questions. If you play just one game, you can accomplish everything, which has to be satisfying.
At some point, many people are just tired of their game, and jaded, and frustrated with game changes or development, and perhaps they stop playing anything at all. Never stop playing! Life is so rich in virtual worlds. You can be the very best you, no limitations. How great is that?
Playing Multiple Games
Keeps all games fresh.
Staves off boredom or dissatisfaction with any particular game, and gives you a broader perspective of what is possible.
You can keep the game client up to date so a long download doesn't discourage you from logging in to play when the impulse hits.
Often when you've been away for awhile you don't recall how to play your class, or your inventory is packed with (stuff) you don't know what to with. You're right up to date with game changes so it's always a familiar place to play.
There are a wealth of games available, so may different worlds, so much imagination in creation and care by the game makers to create a place for you to wander, it's a golden time to be able to try so many styles of play.