If someone have not tell you this yet, power levels does not tell you whether you will win or not. 

I check power levels in arena to gauge whether my Serris can strip enemy buffs or not. So, it is very helpful for me. 

Gives you an indication of their team comp and strength, pretty much. Low team power (<140k) generally means speed nuke team, medium team power is generally a mixed bag (between 140k and 200k), and high team power usually means resist/defence or pure defence teams (>200k).

This exactly. I look at both the champs and the power to determine what type of team I'm facing, whether my team can take em, and how long the fight is gonna be. I'm the 200k pure defense team myself, so some battles against a great defensive team can get a bit long. lol