In theory, the widgets can directly reference the memory that they are manipulating by holding points to the data, rather than copying the data back and forth, but more often than not, it doesn't make sense to use the same data format for storage and for editing.
Take strings of text for example; the best way to store a string is as a character array; whenever you need to send the text anywhere, over the network or to a display, you can simply read from the first character until the last one, and each one can be transmitted as-is. For example, "Hello World" can be stored as the string length followed by each of the characters:
This is not a good way to store a string that is being edited; however, for editing, it's best to have some buffer space around the cursor to avoid having to copy large amounts of data back and forth as the user types and corrects themselves. For example, if the user places their cursor right after the word "Hello", the same array might...