SpaceX designed the Falcon 9 for reuse. They have several iterations, starting with the 1.0 design (with the tic-tac-toe grid of engines).
The switch to the 1.1 design with the Octaweb, was distinctive and was finally able of landing.
The current model is Block 3, 4, or 5 (Block 3's have all been flown, and Block 5 is about to fly, end of April as I write this).
Experimentally, SpaceX determined that they could affordably refly a Block 3 or 4 models a maximum of 2 times. They took the lessons learned and rolled them into what they hope is their final version, Block 5.
Thus, when a booster is on its second flight, it is not going to be flown a third time. Thus the question is, what to do with second flight boosters?
The CRS-14 booster is on its second flight, it is core B1039 last flown as CRS-12.
Mostly they have been running landing experiments trying to see how efficiently they can land, with minimal fuel required.
They have tried to spin the stage up, and then recover from the spin, with minimal fuel usage on one of the Iridium flights which made a spectacular pattern in the sky.
What is interesting is that they are not trying to land at LZ-1, but likely because they want to try another exterme landing approach, and digging a hole in the Florida coast goes over poorly with the FAA, Air Force, and NASA.
A commentator noted that if what I answer is correct, why still mount the grid fins and legs on the boosters.
In the case of the Block 3/4 boosters, the fins are aluminium (The Titanium fins are a feature of the Block 5 booster design, that was tested on a couple of Block 3/4 missions (Like the Falcon Heavy demo flight side cores) and thus not needed for Block 5. The legs on Block 5 are different as well, so no need to stockpile older models). Finally, in order to really test the landing aerodynamics, you to test like you fly, so they mounted the landing equipment to make for a better, more accurate flight test.