At the beginning you can be happy with a good team that works well but as you progress maybe you will want to try different things with different troops.
Besides leveling them increases the power of your kingdoms.
http://community.gemsofwar.com/t/kingdom-bonuses-guide-updated-for-1-0-8/
As for team building, maybe these guides might be helpful?
http://community.gemsofwar.com/t/pvp-guide-for-beginners/
http://community.gemsofwar.com/t/team-building-guide/
http://community.gemsofwar.com/t/guide-to-farming-1-0-8/