Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Strong poke potential
Ezreal is more known as a "Blue build" ADC but also as a strong AP Mid carry.
But what if you try to break the meta ?
First of all ... why should anybody play Ezreal an an AP support ?
Its not easy to answer this question.
First of all you need to be crazy enough to try new builds and new positioning like in this example.
Ezreal as support can be realy strong in the early, mid and late game if your team cooperates with you.
As a main part you should try to explain your ADC that you won't "steal" any farm.
You have a large amounth of gold income with Spellthief's Edge. Also you have high damage in the early game ( first 3-5 minutes) to poke down the enemy ADC or support. This sould help your ADC to concentrate more on farming insted of fighting.
On the other hand you take some kind of the enemys ADC farm because the enemy carry needs to concentrate more on dodgeing your skills.