I thought about comparing phonetic and manual languages to computer data entry methods: the mouse and the keyboard. Naturally I will take English as an example phonetic language and ASL as the example manual language. The same ideas apply to other languages.
I speak different phrases to different kinds of audiences. I try to be precise in how I speak. When I speak, I am thinking of how the words will go together to correctly form word pictures.
Using the keyboard, I have to think of how to appropriately tell the computer how I want it to present my ideas. I type this sentence using the letters. I change how the letters are presented using SHIFT, Ctrl-B, or Ctrl-I. I can also use the Page and Arrow keys and the Alt or Ctrl keys with certain letters to navigate. The keyboard is very flexible in how I can tell the computer precisely what pictures I want to make.
When I speak ASL, I do not consider how to form word pictures. Instead I consider how to form pictures using pantomime and classifiers. Then I use words to add a little more color to them.
The mouse allows me to directly present my ideas on the computer. Then all I really need the keyboard for is to add in a few details.