Both can serve a purpose, but keyframes are probably added more often and provide more utility.
If you add a frame, you are simply extending whatever existed in the frame(s) preceding it. You can add frames within a timeline tween if you need to extend the number of frames it involves and it adapts to take a share of whatever the tween is. You can have a layer of actionscript that only uses the first frame for code, but if you extend it the full length of the timeline using regular frames (simply insert a frame at the end), anything along the timeline can access that code from any frame it covers.
If you add a keyframe, you add an opportunity for change or control... a junction where you can introduce code or new objects or a change in the state of an existing object.
this is how it works, if you are animating with the classic way you need the keyframes to set a position or a change, plus with the key frames you can add anchor points, actions, comments and many other things. Then yes u need to create a frame to place a key frame but if you try to create a keyframe with no frame, flash will create the frames for you
that is a fine question.
only add keyframes (F6) when you have an object on a layer that that needs to have some property (alpha / positiong) changed.
add regular frames aka static frames when you need to extend the amount of time in a layer.
lets say you have a really basic animation of a circle jumping from the left of the stage to the right. it will be on the left for 10 frames and on the right for 10 frames.
lets suppose this circle is already a symbol and it is in a layer with only 1 frame and on the left of the stage.
select frame 10 and add a regular frame(F5).
now frame 1 has a keyframe (black dot) and the new frames (2-10) are regular frames (grey).
drag the playhead to frame 10 and move the circle to the right
drag the playhead back to frame 1
oops the circle changed its position back in frame 1!
an object can only be selected in a keyframe regardless of where the playhead is
do edit > undo until the circle is back on the left
now select frame 10 and add a key frame (f6)
now select the circle and move it to the right of the stage.
drag the playhead back to frame 1.
The circle should be on the left of the stage in frames 1-9 and to the right on frame 10.
if you test your movie you will see that the circle is left most of the time and blinks really quick on the right.
We want it to be on the left and right equal amounts of time
select frame 20 and insert a frame (F5)
Now the new regular frames have exteneded the timeline.
Things to take away from this experiment
-key frames are only needed when something changes
-a keyframe allows you to change an object's properties without effecting those properties in previous frames
-regular grey frames simply mean that the object in the previous keyframe has not moved or changed any properties... it is infact remaining static or stationary.
regular frames should really be thought of as "adding a pause" to your animation. they are where nothing happens.
You really want to wrap your head around this concept as it is crucial to moving forward and creating neat purposeful work.
please feel free to ask for more clarification