The function of an unquoted backslash in a command in the shell is to escape the next character. Another way to say that is that the backslash removes the special meaning of the next character, if that character has a special meaning.
The newline is a command terminator, just like ; and a few other characters. The backslash removes that function from the newline, so that the command may continue on the next line. This is, by the way, why you can't even have a space character after the backslash at the end of a line that needs to be continued, as the backslash would then not escape the actual newline.
Your proposed code would not be parsed correctly by the shell. Instead it would try to run each line as individual commands.
Another approach that may possibly be useful under some circumstances, if your shell has support for arrays (as bash, for example, has), is to store the command in an array like so:
mycommand=( myapp -a 'foo foo' -b 'bar bar' -c 'baz baz' -d 'qux qux' bar )
The newlines in this array assignment just functions as separators between the array elements, just like the spaces do. Elements that needs spaces embedded needs to be quoted, as shown in the example here (and just like they would have needed to be quoted on the command line in any case).
To run that command, you would use
"${mycommand[@]}"
Note that the double quotes are essential. They ensure that each element of the array is individually quoted.
Alternatively, use an array only for the arguments:
myapp_args=( -a 'foo foo' -b 'bar bar' -c 'baz baz' -d 'qux qux' bar )
and then:
myapp "${myapp_args[@]}"
myapp -a fooooooo1, then the next command-b foooo2etc. Since there is no command named-b, this will fail.