Why do some kits come with transparent instrument panel?

It depends on the manufacturer of the model.  The 1/32 Trumpeter Su-25 I'm building came with a clear instrument panel and a decal...but the detail is on the top of the decal, and they even tell you to put it OVER the clear panel.  That's a head-scratcher.  Why make a clear panel if they tell you to cover it up with a decal of the entire panel?  Didn't use any of the in-box cockpit parts though, so it didn't matter, but it definitely made me question what their point was in doing that.

The 1/32 Tamiya Spitfire Mk. XVIe I have built a little bit of, on the other hand, had me baffled at first with their clear instrument panel parts which they wanted me to apply decals to.  I looked at the decal sheet and saw little groups of black dots, which made me curse Tamiya at that moment, thinking "What good does a bunch of black dots do me?!!  I could just paint those!".  But then it hit me...Tamiya put the instrument faces on the BACK of the decals, so they could be applied to the back of the clear parts and use their own adhesive.  So far I have used every cockpit part in that kit, and the instrument panel looks pretty convincing with those decals behind the clear panels.  The clear parts then attach to the backs of the instrument panels, so no masking is necessary.

Don Stauffer

...it is simple to trace the existing panel on sheet styrene, cut it out and apply the decal to the new panel sandwitch. 

I've done something similar, to dress up older kits, like Monogram's TBF.  I wanted the IP to have some relief.  So I photocopied the decal sheet and used a copy of the panel decal as a template to make a panel of "bezels" with some punches, and then I sandwiched another copy of the panel between that and a second piece of styrene sheet to back it.  Just for fun.

Years Ago. I had a Airfix 1/24 J.U.87G. It had a three layer panel. The idea was to trap the decal in between the layers.

Thank you Stick and Don.

What an interesting question. I've noticed that over the years, and been to ditzy to question why.

So thanks for the question ......and the answers.

Do not use that decal as a decal.  Glue it to the back side of the instrument panel.  I use that window material which dries very clear, like Testors or Micro Scale.  Then, after it is completely dry, I use a toothpick to put a drop of clear lacquerover the instruments to add extra gloss.

If one insists on using as a decal, and there is no flat plastic panel, it is simple to trace the existing panel on sheet styrene, cut it out and apply the decal to the new panel sandwitch.

Usually the decal goes on the back with the instruments lined up to show thru the lenses. Then you paint the front of the panel in the appropriate color(s).