Why
You need power to:
- Level up and enhance your league
- Get more air drops
How
To find the best cards to power up, we need to find the best CP(collection power)/usd ratio.
This means we get the most power for each dollar we spend.
1: Go to https://peakmonsters.com/
2: Go to buy section by clicking on 'BUY' tab or navigate to https://peakmonsters.com/market
3: Click on 'COMPARE' button.
4: Choose 'Best CP/Cost ratio' from the dropdown.
5: On this list you see the best CP/Cost ratio cards for this moment.
This information is based on Crypto Llama's video. 
Check his video for more info about collection power.