So what's the pool? Benefits of adding to it?

Example:

In the beginning, all of the 5 tokens were worth $1000, so the pool size was $5000.

Now, say all other 4 tokens remained at their old value ($1000 / token), but STA did a x10, so STA value in pool is now worth $10,000.

That's much more than 20% of the pool, so the balancer must balance it out.

Note that now the pool size is $10,000 (STA) + $4000 (other 4 tokens) = $14,000.

So, to balance it out, each token should now have value of $14,000 / 5 = $2800.

To reach this goal, the balancer must sell the excess STA, and that would be $10,000 (current worth of STA) - $2800 (the new target worth of STA) = $7200.

The balancer sells the excess on various exchanges to which it has a trading bot interface to. Currently, it's only Uniswap.

Note that by selling, it performs transactions, and thus burns tokens, so the supply also decreases!

So, now that the balancer has sold the excess STA, it obtained $7200 dollars (say, in DAI coins).

Now, to balance out the other 4 tokens, it must buy $7200 / 4 = $1800 worth of each token.

So, it buys $1800 worth of ETH, $1800 worth of LINK, etc.

In the end, it's all balanced at 20% for each token - all 5 tokens are now worth $2800 dollars each!

And the total pool size remained at $14,000 dollars - so no value was lost!

Plus, you also get the trading fees, which are added to the pool!

On top of that, Balancer is now issuing BAL tokens for all liquidity providers!

You can check what it is all about here:

https://defipulse.com/blog/balancer-start-liquidity-mining-and-earn-bal-tokens-today/

That's the magic of the balance pools.