Is the little piece of foil tape stuck to the optical drive and fan shroud important?

I believe Apple used conductive foil tape at that location, but if you don't have any you can just leave it without any tape.

The reasoning for use of metal foil may be to reduce odds of electrostatic

effects inside the case of your MacBook; the fan housing may create some.

So that foil may be necessary at least from an engineering standpoint.

..I've seen rolls of metal foil tape, usually aluminum; some thin & others, thicker..

An electronic supply shop may be able to determine the sort of product needed.

I think @K Shaffer's explanation is the correct one here. While I tried to use conductive tape whenever possible, sometimes I had to just use some Kapton tape over the original Apple foil tape just to make sure it wouldn't come loose and get lodged in the fan or short the board. There were times when I didn't have tape and never noticed any issues. I'm not sure if this was the laptop, but I think one laptop had the Top Case/Keyboard make contact with the foil tape.