Keywords are useful in contexts where a program or procedure wants to be able to accept a large number of optional arguments without making its interface unmanageable.
To illustrate this, consider a hypothetical make-window
procedure, which creates a new window on the screen for drawing into
using some graphical toolkit. There are many parameters that the caller
might like to specify, but which could also be sensibly defaulted, for
example:
If make-window did not use keywords, the caller would have to
pass in a value for each possible argument, remembering the correct
argument order and using a special value to indicate the default value
for that argument:
(make-window 'default ;; Color depth
'default ;; Background color
800 ;; Width
100 ;; Height
...) ;; More make-window arguments
With keywords, on the other hand, defaulted arguments are omitted, and non-default arguments are clearly tagged by the appropriate keyword. As a result, the invocation becomes much clearer:
(make-window #:width 800 #:height 100)
On the other hand, for a simpler procedure with few arguments, the use
of keywords would be a hindrance rather than a help. The primitive
procedure cons, for example, would not be improved if it had to
be invoked as
(cons #:car x #:cdr y)
So the decision whether to use keywords or not is purely pragmatic: use them if they will clarify the procedure invocation at point of call.