> What evidence is there for that? […] Why is there never any proof or evidence required for saying an artifact has a religious purpose?
From a related Wikipedia page:
It is assumed that the Golden Hats served as religious insignia for the deities or priests of a sun cult then widespread in Central Europe. Their use as head-gear is strongly supported by the fact that the three of four examples have a cap-like widening at the bottom of the cone, and that their openings are oval (not round), with diameters and shapes roughly equivalent to those of a human skull. The figural depiction of an object resembling a conical hat on a stone slab of the King's Grave at Kivik, Southern Sweden, strongly supports their association with religion and cult, as does the fact that the known examples appear to have been deposited (buried) carefully.
I'm sure you can find more precise details if you dig deep enough.
> Which sun-god?
You can read more about this cult here[1] and here[2].
> This is a calendar. It's proven to be a calendar. We know how it works as a calendar. It's a tool for predicting celestial events.
Do not project the hyper-rationalistic character of our current society to societies and civilizations of the pre-industrial era. People in the ancient times were not as rational and utilitarian as we are. Everything was permeated by religion, superstition and magic.
[1]: https://www.scirp.org/journal/paperinformation.aspx?paperid=...
[2]: https://www.vice.com/en_us/article/wnja9z/scientists-reconst...