Vito basically creates an alibi for the later murder of Carlo by bringing him to the center of the family and trusting him with important information. It keeps Carlo nearby, so that they can get at him. It also gives the appearance that the Corleones don't suspect Carlo.

Also, the meetings Carlo attends are partially a sham, even though Tessio and Clemenza are present. The real planning is being done between Michael and Vito, and then one or both of them will meet with Tom, Neri, or Rocco as needed. The circle including Carlo, Tessio, and Clemenza gets only the information that Vito and Michael want them to have.