The Western Provinces Achievement Diary gives rewards by tier, and the biggest perks come from the hard and elite tiers. Here’s a clean breakdown in HTML table form:

Tier Rewards
Easy Western Banner 1, 25% more chompy birds while hunting, 25 free ogre arrows per day, and an antique lamp worth 2,500 XP in a skill above level 30.
Medium Western Banner 2, 50% more chompy birds while hunting, Crystal saw holds twice as many charges, 50 free ogre arrows per day, and an antique lamp worth 7,500 XP in a skill above level 40.
Hard Western Banner 3, upgrade void knight top and bottom for better prayer and damage bonus, access to the hunter master’s private red chinchompa area, 100 free ogre arrows per day, one free Piscatoris fishing colony teleport per day, crystal halberd purchase unlock, and an antique lamp worth 15,000 XP in a skill above level 50.
Elite Western Banner 4, 100% chompy bird appearance chance, chance at the chompy chick pet, increased Slayer points from Nieve/Steve, 150 free ogre arrows per day, one free resurrection per day at Zulrah, and unlimited Piscatoris fishing colony teleports.
These rewards are summarized from OSRS guide sources that list the diary tiers and their unlocks, including the banner upgrades, daily benefits, teleports, and lamps.