Having a few accounts allows you to use different settings and trading styles.
Let's say you trade 2000$ and split them in 5 accounts.
500 you put on gold because gold performed in the best way and you think if this goes well it wil be good profit for you. You don't want to spread the lot sizes across different markets. All in gold on this account.
800 you put on a combination of Gold, GBPUSD, EURUSD, USDJPY - this one you lower the risk% of each market to 1 - 1.5% per trade - because you don't want to risk too much and this combination will be probably the most stable account
300 you put on GBPUSD - because you just like the market and you want to trade with 3% of the capital on it, 200 you put in a very low risk conservative strategy.
And the last 200 you put maybe on gold but really aggressive settings (higher risk higher reward) - you go crazy and set 5% per trade or something kamikaze like that
Now you have an interesting portfolio. If all goes well you will be well in profit. If some markets don't perform so well - the diversification of risk will allow you to pass the bad times with relative small drawdowns and let the good periods flow profit into your capital and if the high risk ones happen to execute your way, then you got rich and lucky.
The point is - you can use different strategies and trading styles.
If you want to use just one account, use just one. But when you trade more markets, you lower the risk.