  Magnetic fields affect the local structure of the photosphere of stars. They
can considerably influence the radiative properties near the optical surface,
flow velocities, and the temperature and pressure profiles. We aim at
understanding qualitatively the influence of small magnetic flux concentrations
in unipolar plage regions on the centre-to-limb variation of the intensity and
its contrast and on the shape of spectral line profiles in cool main-sequence
stars. We analyse the bolometric and continuum intensity and its angular
dependence of 24 radiative magnetohydrodynamic simulations of the near-surface
layers of main-sequence stars with six different sets of stellar parameters
(spectral types F to early M) and four different average magnetic field
strengths (including the non-magnetic case). We also calculated disc-integrated
profiles of three spectral lines. The small magnetic flux concentrations formed
in the magnetic runs of simulations have a considerable impact on the intensity
and its centre-to-limb variation. Spectral lines are not only broadened owing
to the Zeeman effect, but are also strongly affected by the modified
thermodynamical structure and flow patterns. This indirect magnetic impact on
the line profiles is often bigger than that of the Zeeman effect. The effects
of the magnetic field on the radiation leaving the star can be considerable and
is not restricted to spectral line broadening and polarisation by the Zeeman
effect. The inhomogeneous structure of the magnetic field on small length
scales and its impact on (and spatial correlation with) the local
thermodynamical structure and the flow field near the surface influence the
measurement of the global field properties and stellar parameters. These
effects need to be taken into account in the interpretation of observations.
