I present results of fully self-consistent synthetic dust polarisation maps of simulated molecular clouds (MCs) created with POLARIS (Seifried et al., 2019+2020a). By incorporating noise, we assess the reliability of polarisation observations from e.g. Planck and BlastPol. I will discuss how strong projection effects affect the observed orientation of magnetic fields. Furthermore, I will show that polarisation observations of MCs mainly probe their dense parts up to AV > 1 (~1000 cm^-3). I will show that magnetic fields become perpendicular to (column) density structures above N = 10^(21-22) cm^-2 and n ~ 1000 cm^-3 and discuss how this orientation flip relates to the question where and when gravity starts to dominate over magnetic fields. Finally I briefly shed light on how magnetic fields influence the chemical evolution of molecular clouds by making their structure more fluffy and diffuse (Seifried et al., 2020b) as this has a direct impact on both the amount of H2 and CO-dark gas. In addition, I will discuss how the switch of the magnetic field orientation is related to the chemical state of the gas.