Beam in roof has cracked, is this serious?
I would replace it with a new piece of similar sized and type of material. Should be really straight forward to remove the broken one and place the new one. 

Sistering a new one on against the existing one is another option if it is not possible to get at the ends of the existing member. 

It is indeed possible that these play a minor role in overall structure of your house. They may have been installed at the time of the building construction to keep the ridge board in position as the rafters were cut and nailed into place. But even that said they do provide some triangulation stiffening of the roof structure in the case of horizontal stress placed on the building structure by winds.