I'm going to borrow from an old wood chuck tongue twister for this:
How many fish
can a fish finder find
if a fish finder can find fish?
A fish finder will find
all the fish it will find
if a fish finder will find fish.
Call it what you like fish finder, depth finder, fish locator, sonar unit. No matter what you call it it's a way to learn the depth, bottom structure and location of the fish underwater. There is one thing about looking into a lake, you don't know how deep it is unless you can see bottom in the shallows. Sometimes on a windy day even that isn't possible.
How does a fish finder work. It sends a sonar signal from the transducer to the bottom of the lake and receives an echo back to the transducer that converts it to an image which is then recorded on your screen. If there happens to be a fish in the way it also marks that reading on the screen. I always keep my Lowrance on fish mode so it draws a fish shape and also tells me the size of the fish. All helps are good while fishing.
So when do I depend on my fish finder. There are two times I use it extensively:
1. You need to know what depth the water is where you are running while trolling a lure behind the boat . It's nice to see a fish on the fish finder but sometimes they come from the side and you get strikes when least expected. I can stay at the constant depth and keep the lures in the catching zone for a longer time.
2. Another time is when searching the bottom for structure that are holding fish. I can remember several time when the fish finder told me there was structure under the boat and fish were on the structure that I have caught fish from the spot. One time I caught an 18" rainbow trout from that place I would have never dropped my bait to the bottom. There was another time I sat over a school of fish in an area 50 foot from shore and caught a variety of fish from the spot. The people on shore couldn't believe that I kept pulling in the fish one after the other.
So I use the tools I have to make myself a better fisherman.