Because it offers people a competitive element to gaming. Looking at the popularity of COD, Starcraft and MOBA games, it's evident that an awful lot of people enjoy the competitive aspect. People who say that multiplayer is unimportant or dumb really don't understand what games are all about: fun.

To put it simply, it matters because people enjoy it.

@bolded what??

Like i said for gamers it adds value, features, more game time and also interactions with friends. When people love a game they want to keep playing it, and if it has good gameplay multiplayer is never bad. 

Isn't that a major part of fps?

So it really isn't important.

multiplayer was always a big thing for me but nowadays i don't sit with some of my 30 years old friends in a room to play videogames with them so it is really great for me that i can play with them online.

many of my real life friends live hundreds of kilometers away so even if i would like to sit with them together on a couch it wouldn't be so easy i am in the lucky position that many of my friends are also still "hardcore" gamers who invest a lot of time in this hobby even with that age. there is always someone online to play with when i go online with my 360 and i don't even have friends in my ps3 list or i would play too much online...

i'm really happy that next gen seems to get also more co-op experiences because i love co-op games. i played a game like the last of us maybe 25 hours but i will play a game like destiny 250 hours. even if it wouldn't have any story i would have a lot of fun with it just because it's great to play with friends.

You know, I tried to play golf a few times. It wasn't that good of an experience. I don't understand why it matters and think we should get rid of it. Actually, anything I don't really like, we should get rid of. Table top games? Boring and static and they require I drive somewhere far away. Why do they matter?

Seriously, why do people complain about things that other people find fun?

Here's your reason. The TRUE reason.

If you look at the biggest selling MP game (on consoles) it seems MP only matters to about 17% of the console gaming market at most. That's a sizable chunk of the market to be sure, but still only a small minority of gamers.

MP certainly doesn't matter to me. An immersive SP>>>>MP any day.

It might matter to some, but the only really cool online experience I had this gen was with Journey. 2nd place was Dark souls although the online was just as much getting in the way as adding to the game.
It's fun for racing games now and then, mostly out of convenience in having someone else choose the tracks and cars. A glorified random play mode to keep things interesting after finishing the game.

With kids, work and timezones getting in the way, meeting up with my friends online is pretty much impossible nowadays. I rather play local multiplayer with the wife and kids, and when family or friends come to visit. Online is not very engaging.

I'm scared to play online. I fear getting owned by people on COD. I don't want to talk on a mic with people. I'm anti-social and want to play with myself.

Because we're a social species with a spectrum of social needs such as co-operation, competition, attention, acknowledgement and pride, and because a real human is far superior to an artificial AI.