Apple Wi-Fi assist is designed to automatically switch your device back to 4G if the Wi-Fi is deemed too slow. Being in Australia with the pathetic roll out of the NBN from both governments I assume you will be on 4G a lot.
This will lead to an increase in your data usage where you are likely to go over your data allowance, and get hit with a large bill from your telco.
At Qbit we recommend you turn off this feature that is now enabled by default on the iOS9 update.
Open ‘Settings’ and select ‘Mobile Data’. Scroll all the way down to the bottom until you find the ‘Wi-Fi Assist’ toggle and turn it off! Easy and money saving.
Android phones have a download booster that combines Wi-Fi and Wireless to give you faster internet, this feature is not enabled by default.