Arches can span much wider openings as the voussoirs which often constitute arch structures are easier to transport to the work-site than a very large beam would be.
Arches are able to carry much greater loads than beams as the different stresses are focused on the supports from diagonal thrust, instead of within the material.
Although the transferal of the dead and live loads to the supports means that arch bridges need very large supports and/or buttressing, in bridges with sequential arches, this is not the case bar the arches at the extremities, because the horizontal forces in the supports of adjacent arches cancel each other out.
