What is the purpose of this disk over a Samtec connector?

That orange disk represents a pick-and-place sticker—it provides something for the pick-and-place machine's vacuum head to grab onto. Without this, you'd have to assemble the connector by hand.

Here is an example photo of a similar Samtec SS4 with the vacuum pick and place sticker in place:

This hasn't been explicitly mentioned yet, but you got a disc because you asked for one.

It's a Polyimide film pad, and specified by the last code in your part: -K. The purpose of this pad is so the vacuum pick-and-place machine has something to hold the component on. It needs a flat surface big enough for the vacuum head, as already shown by Jeroen3 and DKNguyen. For manual manufacturing, you don't need the film pad. But some automated lines can't pick up the parts if there's no flat surface provided.

QSH-030-01-F-D-A-K has the pad, QSH-030-01-F-D-A does not.

030-01 says how many positions and the lead style. The other codes:

• -F: 3u plating
• -D: double rows
• -A: alignment pins
• -K: Polyimide film pad

See the QSH configurator. Find your new, padless STP files there too.

Polyimide (also known under the brandname Kapton) is stable for a wide temperature range and a good electrical isolator, making it ideal for this purpose.

It is a place for the pick-and-place machine's vacuum head to grab onto.

Taken from Adafruit