The ladder provides the bondholder with a degree of freedom and some liquidity to take part in possibly improved interest rates in the future.
The ladder strategy distributes your funds uniformly among bonds with various durations. For example, if you have $10,000, you buy one bond with a duration of one year, one bond with a duration of two years, etc.
If the interest rates go up when the shorter-duration bonds expire, you will be able to reinvest this money with a higher coupon rate (of course, keep in mind that your longer-duration bonds would have fallen in price).
Can You Sell a Bond for Less Than the Price You Paid for It?
What is a Bond Ladder?
How Do I Structure My Bond Portfolio?