I have a fishfinder, but I don't fish.
There are times when layers of floating plants will give you a false shallow depth reading. There are also times when the water is too deep for a reading. In both cases the fishfinder will show you what's going on.
The fishfinder shows a graphical history of depth, so you can extrapolate from the reading you got a minute ago. The fish finder can also show soundings at multiple depths simultaneously, so you can see the plant layer as well as the true bottom.
A simple display with just a number is nice, but I wouldn't want to be without the details that a fishfinder provides.
And lastly, the fishfinder is fun when you have kids aboard. They love watching for fish.