Why does ChessMaster recommend 24. Rg3 in this position? I reached this position when playing against ChessMaster's cyber personality Laura. The engine recommends the line starting with 25. Rg3:

[FEN ""]
[White "Bad_Bishop"]
[Black "Laura"]
[StartPly "48"]

  1. d4             Nf6           
  2. Nf3            c5            
  3. d5             d6            
  4. c4             Qa5+          
  5. Bd2            Qa6           
  6. Qc2            Bg4           
  7. Nc3            Nbd7          
  8. e4             Bxf3          
  9. gxf3           h6            
 10. h4             e6            
 11. O-O-O          Ne5           
 12. Be2            exd5          
 13. exd5           Be7           
 14. f4             Ned7          
 15. Rhg1           Kf8           
 16. Rg2            Ne8           
 17. h5             Bf6           
 18. b3             Qa3+          
 19. Kb1            Bxc3          
 20. Bxc3           Ndf6          
 21. Rdg1           Rg8           
 22. Bf3            Qa6           
 23. Qe2            Qb6           
 24. Re1            Qd8 25. Rg3 b5 26. cxb5 Nc7 27. Qd3 Rb8 28. Ba5 Qc8 29. a4 Qb7 *


To me 25. Rg3 is not intuitive and I did not consider it all in choosing a move. Granted, in the line that ChessMaster recommends, black throws away a pawn, but my question is, what is the purpose behind 25. Rg3 and why is it the best move?
All of White's other pieces are placed well except the Bishop on f3.  If Black moved the a8 rook back and forth then White would probably like to play Rg3, Bg2-h3, and maybe Rg3-e3.
Simply put Rg3 increases the scope of the rook and the bishop.