Tong-
The name Fishfinder is a bit misleading because while yes it does locate fish,
it's value is more often in terms of letting you know the depth and bottom structure of the waters you are fishing. It also lets you know the depths at which the fish are located. So it helps you to be aware of your fishing environment and fish most effectively, which in turn will improve your catch.
But it is a mistake to think that every fish you see on the FF is a potential catch. Sometimes they are not hungry, they are different species of fish, or baitfish, or even readings of other suspended materiel that gets translated as a fish. Admittedly, there have been a few occasions where I did spot the fish on FF, adjusted my presentation depth and caught the fish. But in 99% of the time you do not see the fish that you catch. And the fish you see on the FF won't hit your presentation.