People are talking about two types of fartleks here.
Calling a fartlek "time intervals" and saying it is not a fartlek, is like saying vegetable lasagna is not lasagna because the original was made with meat.
Both are fartleks. One is the original, the "pure fartlek". The other is a "structured fartlek".
You can do all the speed play you want in the second type, especially when running on hilly ground. All you do is control the time you will be spending running fast and nott so fast.
OP, fartleks can serve a number of purposes.
Some types have already been mentioned here. Peter Coe, for example, suggests a type of hill workout done as a fartlek, where you run on undulating/hilly ground and speed up on the uphills. This can have short hills where you go all out and long hills, where you must pace yourself.
You can, for example, do a fartlek on top of tempo pace. If you are a runner who competes for place, running at tempo pace and then doing 30s-1min surges (coming back to tempo pace for "recovery") will prepare you to surge and to respond to surges in races.
You can do 10x1min very fast and jog the recoveries and you will be doing almost the same as a 300m+ track workout.
You can do 3x8min for tempo work.
The great thing about fartlek is that you can be very creative.
But you have to be honest with yourself and run hard. Like the other post said, fartleks are not supposed to be easy workouts.