Why are archaeologists concluding that this was used for religious purposes? Why is religion always the central theme in those old findings?
The buildings themselves are the biggest proof. No roof, inconvenient and small entrance, holes to let the soul escape. The carvings found at Göbekli were also found on other sites which were associated with religion and death. One example would be the carving of vultures and headless bodies. Vultures which are associated with death and the ritual of excarnation: the floor of the buildings were made waterproof and sloped to allow the draining of bodily liquids. We know that people in that area would bury bodies without their head or even reopen the grave to remove the head. They also have been known to put plaster on skulls and keep the heads around.
The strict functional differentiation of society into subfields such as politics, education, science, law etc that characterises modernity, emerged only over the last couple of centuries. In the past they were really much more intermeshed. What we mean by religion today has relatively little relevant to ancient societies.
Not the stratification of professional roles we see today (which is pretty staggering) still breaks down not too far from these categories. The US BLS EEO-1 Job Classification Guide's top level breaks down to the following major classifications, each with the indicated number of subclassifications:
1 Exec/Senior Offs & Mgrs.
33 First/Mid Offs & Mgrs.
266 Professionals
57 Technicians
21 Sales Workers
64 Administrative Support Workers
132 Craft Workers
120 Operatives
34 Labors and Helpers
91 Service Workers
From Integrated Public use Microdata Series, classifications by year:
https://usa.ipums.org/usa/intro.shtml https://usa.ipums.org/usa-action/variables/OCC#codes_section
1880: 227
1920: 587
1930: 283
1940: 236
1950: 269
1960: 302
1970: 444
1980: 505
1990: 513
2000: 545
https://www.reddit.com/r/dredmorbius/comments/3832wx/occupat...
The evolution of society from the stone ages to modernity is characterised by increasing:
1. Functional differentiation. That means we specialise more in smaller and smaller fields. Even 100 years ago westerners would typically have grown some of their food themselves, and might have played a role in building their own homes.
2. Voluntarism. That means that we choose our field of work ourselves. In the past our social station made that decision for us. If our father was a farmer, we'd probably be a farmer too, if daddy was a slave we'd be slaves etc. Nowadays we progress through a sequence of choices (which university to go to, which field to study, what company to work for etc) and a sequence of exams and tests that seeks to quantify our ability for a chosen job.
Weren't castes or other social segments strongly associated with certain roles actually fairly strongly established?
The classifications I posted above show that how specialized you see work depends much on how the classifications are defined. My sense is that periods of rapid change in employment structure lead to increases in classifications. E.g., the 1920 set. Which actually covered 1900 - 1920. See Vaclav Smil's recent articles on the innovation of the 1880s.