So my layman understand (and having seen it in practice) understanding of A is as follows;
So higher A is better when coins are at peg. Meaning with high A the pool assumes all the assets are as close to 1 as possible. Low A is the opposite.
So the high A of y pool, has essentially allowed arbitragers to keep getting DAI for 1, when in fact they should have been paying 1.01 ~ 1.02. So by bringing A lower, it makes DAI more expensive to buy. But, since y pool is incredibly low on DAI, it also makes DAI nice to sell to it. This will keep the pool more evenly spread between assets, which I generally think is positive for all LPs, will remove a source of cheap DAI from the market (good for DAI). And create a healthier balance of assets.
Normally we would not have noticed this, but given the large influx of DAI from yETH it became very apparent that we need to suggest a change, since as we expect more money to flow in, we will see this in other areas as well. An example is Binance adding USDT staking via curve. Their deposits will be one sided, if they were to use a lop sided pool (like y), they would have large slippage and this could set them off from using it in the future.
From my observation I think since curve is turning into almost a savings account / trade platform / stable coin meta basket, this change is important.
If you look at mStable for example, you can essentially assume A is 9999, this is why one side of the basket is often completely empty, and they are left with 1~2 assets. We want to prevent this and make the usage and adoption of yUSD (output LP token from y pool) more widely used and adopted.