So on the picture below you can see that already on level 4 friendship you get extra 700 health which is very much achievable and a nice additional bonus.
On level 6 is the dream of archers with +150 critical damage, but of course harder to get to.
You can increase friendship by:
- Energizing the tree of your friends
- Completing dungeons together
- Giving each other Roses (1 rose, 11 roses, 99 roses or 999 roses)
- Melding Sylphs (in the farm)