I understand the rewards and that there are tiers but what d tiers have to do with winning a CG?
This varies from CG to CG - you need to read the CG descriptions in detail to find out exactly what happens, and sometimes it's not completely described upfront.
Station Trade CGs: 2 CGs each with 5 tiers, each tier gives one station upgraded, for a possible total of ten upgrades. Tiers were evenly spaced, so each station was equally difficult. Hidden effect: market prices of certain goods change after CG completion. Whether that wouldn't have happened if they'd only reached a low tier, we don't know.
Funeral Rares CG: each tier gives a temporary 5% discount on Gutamaya ships, for a possible total of 25%. Tiers were increasingly far apart, so getting a small discount was easy but getting a large discount was difficult.
Eurybia War CGs: 2 competing CGs, side with the highest tier at the end of the week wins, getting Tier 5 ends the CG immediately with a win, no special mid-tier effects other than payment.
Ackwada War CGs: 2 competing CGs, defending side gets partial victory at Tier 3 and Tier 4, attacking side can prevent this by reaching Tier 5 first, various complex combinations of results possible. Tiers were unusually and asymmetrically spaced to reflect the difficulty of the goals of each side
Witch Head AX CGs: each tier defends one system, tiers evenly spaced.