The idea in one sentence
Interest on savings is paid on the balance, and the balance includes interest paid before. So growth speeds up a little each year.
The example
You have 2,000 euros today. You set aside 600 euros at the end of every month. You imagine a yearly rate of 2.0%, compounded monthly. This is a rate to test, not an offer.
After 10 years you would have 82,074 euros. You put in 74,000 of that (2,000 plus 600 times 120 months). The other 8,074 euros is interest.
Where the interest comes from
In year one the interest is only 107 euros. By year ten it is much larger, because the balance it is paid on has grown: the balance at the end of year nine is 73,328 euros, and year ten adds 8,074 minus 6,528 = 1,546 euros of interest in that one year.
What moves the result most
Try the calculator. Raise the rate by one point to 3.0% and the balance after ten years becomes 86,544 euros: 4,469 euros more. Raise the monthly amount from 600 to 800 euros and the balance becomes 108,618 euros: far more, because you put in 24,000 euros more.
In this example, how much you set aside matters more than the rate. That is not a rule for every case; it is what these numbers show.
A caution
A real account has fees, tax and a rate that changes. The calculator ignores all three. Use it to build intuition, not to plan a purchase.
Try this example in the calculator
Not financial advice. All amounts are in euros (EUR) and are examples.