You can use it for many scenarios. I will name 2 scenarios. If I start with ghost as my summoner spell and prioritize early ganks. Ionian would be the choice since I can decrease the cd on my summoner spells. Another thing is when there's not too much crowd control on the enemy team. If you're behind and need a cheaper boot. Ionian will do the job to fulfill that while you can focus on building your core items.