Well as a catfisherman you'll use two speeds,,,Hyper, very fast or what ever you want to call it for running and looking for likely structure.  With this setting you may miss some weaker returns of fish in structure, but your boat wont be able to out run the returns for display.  Now that you have found the likely structure slow down the Scrolling Speed and watch your display for careful inspection.

Think of it like this---if your running down the isle of your super market you may notice that your in the cereal section.  Something Colorful catches your eye and you slow down and you notice it's got a picture of a tiger on it....hmmm I need to check this out you think to your self and you pull out your reading glasses (2X, 4X, 6X, 8X zoom settings on your fishfinder) and now you see that its Kellogs Frosted Flakes ---- Gee I wonder if it's got any Peanut products in it---cant do peanuts as I'm allergic to Peanuts your thinking so you look at the ingredients label on the side, but since you've had 12 cups of coffee this morning everthing is moving too fast, so you pin that box to the floor so you can read the label (slower setting on your scrolling speed still yet) and see that it has no Peanut products  in it

Thats what your Scrolling Speed does for you in a nutshell--oops sorry for the pun  ::