What is the purpose of these roof joists/braces in old house can they be removed
Those at least appear to be collar ties, a very important part of the roof system. Do not remove them. The squarish looking beam running perpendicular to the rafters appears to be a strong back or purlin. It helps unify the rafters so if one takes a load, it is distributed over many. Also an important component, they are held in place by the collar ties. everything would need to be rebuilt with a design by a structural engineer before any of this can be removed.