When you put money in a savings account, what you get back depends on the interest rate on the account. You know for certain what you will earn and there is no risk of losing your money. When you invest, your returns are dependent on how well your investments do and they could go up and down in value.
Why invest then?
Because evidence* suggests that over the longer term, stocks & shares tend to outperform cash, sometimes by significant amounts. Investing is not for everyone: you need to be comfortable that the value could fall as historic performance is no guarantee of what will happen in the future and you should be saving for the longer term (i.e. over 5 years) as with a Junior ISA.
This chart illustrates the point by showing the value of £1000 invested in the Fidelity Global fund we offer versus an average savings account over the last 10 years. You can see that at times the value of the Fidelity fund fell, but over the longer term it significantly outperformed cash.