The main reason you're probably thinking of is *safety* (for use later with food or cooking).
That's not really a problem in your case though because your surface is not porous, and there are no little crevices in it where the clay/plasticizer could get trapped and be hard to remove. (So just wash your baking tray with soap and water well or wipe with alcohol till clean.)
Other factors can apply to metal baking trays and polymer clay though when curing especially:
...One is that if polymer clay is baked *in direct contact* with a very smooth surface (metal or aluminum foil, glass, ceramic, etc), it will have a shiny spot in those particular areas after baking. That's because polymer clay softens slightly when heated but enough to take on the surface texture of anything it's touching.
...The other one is that metals and ceramics, for example, *can* heat up higher than the oven cavity's air temp and therefore darken the bottom of the baking item or just speed up baking from one side.
For both problems, you can just place a sheet of plain white paper on top of the metal tray then place your item on that when ready to bake. Paper has approximately the same surface texture of baked polymer clay. You can even accordion-fold the paper for baking beads, pens and some other items.
Other materials will work too under (or over or around) the clay, and especially when you're trying to "protect" the clay from darkening, etc, while baking.
There's much more info about ways to protect the clay while baking, baking surfaces, safety, and more on the Baking page of my site, if you're interested:http://glassattic.com/polymer/baking.htm
P.S. Polymer clay is not "toxic" in the true sense of that word--well, unless you burned it, passed out, and breathed the thick black choking smoke for long enough to kill you.