Example: Junior stocks and shares ISA
Let's say you've just had a baby. You're getting through the sleepless nights and soggy nappies, and people are approaching you to ask where they can start saving for your new child. Ideally, you'd like any money to be saved for when they really need it, such as to put a deposit on a house or to go towards their time at university.
Say, for example, you were to put £500 into your child's JISA each year including the year that they turn 18, the child could have £15,269.50 at the end of the period, if we assumed an interest rate of 5% and didn't consider fees. In total, you would have deposited £7,600, so you'd have gained £7,669.50 in interest.
Because the money is locked away for such a long period, and interest gains aren't withdrawn, you can make interest on previously earned interest. This is called compounding, and is one of the main reasons it's best to start saving money for things like your retirement early.
If the stock market performs better – say you see growth of 7%, then they could have £18,689.48 by the end of their 18th year. You'd have still deposited £7,600, so a massive £11,089.48 would be interest.
If the stock market doesn't perform very well and you just see annual growth of 2%, then the child would have £11,420.28. This would include interest of £3,820.28.
* This is a fictional, but realistic, example.