rainy days
short term
long term
compound interest
long term investing

If you are setting money aside for far off goals, like sending kids to university or retiring in style you will need the money to grow on itself, helping along to achieve you financial goals.
Here we talk about 10, 20, 30 or even more years in which you do not plan to take the money out and spend it.

As you are facing long time-spans you can also afford to have period’s where your worth actually falls below what you have put into the pot. As long as you can wait for the respective markets to recover you should be fine.

Even better if you are accumulating wealth by putting money aside on regular intervals, as you have the good luck to be buying more parts of what ever instrument you are using the lower the prices fall. See cost averaging.
Also the page regarding asset allocation should be of interest, so you can start figuring out where to put you capital.