Exactly melkor +1 .I actually agree with Dracont. Arena forces you to actually think about your plan, as opposed to just doing whatever you want in arabia and still winning by good macro.
The difference with arena is, that usually if you let your opponent get castles outside your walls, then its way harder to reboom. This means that either you need to really have a good plan for not dying to the FI or whatever push is happening or you need to not let your opponent get the castles there in the first place. If you don't have a good plan certain civ matchups are instant defeats if playing vs a good arena player.
To be honest I've learnt more about arabia gameplay by playing arena recently than my last 2 years of arabia 1v1s have taught me.
In 2014-2015(can’t remember which year) I was asked and practiced arena privât unrated games, with some of the very top players so they can learn and improve many aspect of the games, main purpose for them was to learn how to boom under pressure, and reboom/expend eco, also see strategic point of a map and map control.