Forretress is hard to fit on Rain teams because without Ferrothorn they tend to be weak to Zekrom, Kyogre, and Manaphy, among other things. The advantage of Forry is that it gives you a strong matchup against teams using Ferrothorn.
For example, here's a challenge: make a Rainstall/Rain Balance team without Ferrothorn. Its pretty difficult.
The best I could do was:
Forretress (use HP Ice for Gliscor :) )
Groundceus (Swords Dancer or Perish Song user)
Chansey (Wish Passer or Snatch)
Giratina (Specially Defensive)
Dialga (Scarfed with Outrage for Lati Twins)
Sun can use Forry because it doesn't tend to be too bothered by Zekrom or Manaphy. For example:
is a pretty typical Sun team.