there are two roles for architect:
1) SharePoint Architect (Development)
He is basically responsible for, Mapping Client Requirements, Lead the team of developers, Solution Design, articulating solution risks and barriers, Recommending the Solutions, Plan,Prepare and execute the Project and all top level stuff.
2) SharePoint Architect (Infrastructure)
He is responsible for same thing but from Infrastructure side, what are the client requirement, suggest the Topology of Farm( how many servers), identifying what kind of services applications client needs, authentication requirement, capacity planning etc.
I am giving you the information on basis of what i observed in last couple of projects.