We present some of the results from the ROSAT all-sky X-ray galaxy cluster survey, which is currently the largest well-defined and tested X-ray galaxy cluster sample. We discuss our cosmological constraints and the significance of their discrepancies found between the probes of the large-scale structure and the Planck. Possible sources of discrepancy will be discussed in terms of non-zero total mass of neutrinos, Hubble parameter and cluster scaling relations. In particular we will discuss the importance of the cluster selection by comparing the cluster properties found in a volume-limited sample to a flux-limited one. We will present a new way to understand the large-scale distribution of galaxy clusters through superstes-clusters, which are the objects that will collapse in the future. The well-defined construction procedure of the superstesclusters allows us to selection analogous objects from cosmological N-body simulations. We use the simulated superstes-clusters to further explore their system properties in light of the observational findings.