Can someone please explain why multi-tabling is so important in online poker?

It's in your title lol

I'm trying to explain this to someone that doesn't understand how volume affects anything. I'm obviously doing a poor job explaining, and was hoping someone could do it better.

With a given amount of money you can play it all on one table, or spread it out over several lower stakes tables. Assuming expected win rate is about the same, spreading the money / play across more tables and more hands reduces variance significantly.

Reducing variance in this way reduces risk of ruin, so with a given bankroll you can actually play even more money at one time than you would otherwise.

That's the theory.

In reality the above effect means that a lot of good players play lower, putting more winning players and volume in low stakes.

They crave the lower variance but are all fighting for a fixed amount of money (i.e. the money LOST at those stakes)... so these winning players are all paying more effective rake than they would at higher stakes as they are fighting more reg-v-reg hands. To mitigate the impact on the winrates, they have to put in more volume. A vicious cycle.

Why don't they just move up in stakes and play fewer tables? Because it requires a disproportionately larger bankroll and increases variance.

In the old live-poker days, players HAD to play above their rolls as they had to try and maximise their winrate and / or play in the good games they had access to. That's why the rounders went broke much more often, despite crushing (comparatively) their games.

David L already gave some good answers above. I'd add that "spreading your risk" over multiple tables is somewhat analogous to the way manufacturers have diverse product lines, or stockholders (or crypto enthusiasts) have a portfolio of stocks and shares or altcoins, instead of investing everything into one single product or company.
Of course volume isn't everything, and playing 16 tables might not be more profitable than playing 8, in the same way that owning 50 different crypo currencies might not be better than just picking half a dozen that perform better on average.

Just ask that person if there is any difference in working 1 hour a week or 40 hours.

David Lyons, ArtyMcFly, quaid745: Thank you for your thoughtful answers to my question. I think you guys nailed it eloquently!

I've never been a volume player as a gamestarter, which I've been for a long time. I can tell you about 70k hand stretches where I won at least twice my winrate. I can tell you about losing years. You wanna talk volume? I say put in the time and find out just how bountiful this game can be when it's good, and also find out just how low you can get when it's bad.

This is exactly why I tell people to play for fun and see where it goes. Would you tell a golf enthusiast that he sucks and should quit? I wouldn't; neither would I tell a poker hobbyist that he sucks and should quit. However, I would give these players some salt before I tell them this:

I've been completely obsessed with both games, as well as basketball and women during different periods in my life. The can all mess with your mind in all directions. This is why you need balance in your life between maintaining important relationships, work, hobbies, and whatever else you liked to do in your spare time before poker, golf, basketball, or that attractive member of the opposite sex that you like came along.

You encourage self wisdom. I like it.

1. Raises your hourly (up to a point)

2. Gets you to the long run quicker

Right on!

Poker is about being patient and waiting for profitable situations. If you play 6 tables at the same time then you can wait on 5 and play on 1.

Its not important if you do it for fun and because you like the game. Rather enjoy it. Other option is to do hard work making 8bb/100 at 10NL or 5bb/100 at 25Nl not such a great play day after all. No need to play away life for cents. Keep mind right and play for fun( while trying to get better) is a way to go.