The two frigates to be trashed were built in the mid-1980s to hunt down Russian submarines.
At that time, the former Soviet Union and Britain weren't very friendly. But now Russia and Britain are friends, so there's no need to have as many of the expensive boats around.
The Navy say using old boats as real-life targets is very important to test how good their weapons are.
They strip the boats of all useful equipment before they blow them up. And they get rid of all the fuel and oil in them so as not to damage the environment.