I have never owned an offline wallet before, but I know the dangers involved in storing your assets in an online wallet, your whole coins are controlled by the exchange, the exchange can steal your coins and claim it is a hacked job pretty ugly. I would advice anyone who wishes to have digital assets to use an offline wallet over an online wallet. I don't have any tangible reason to give you why online wallet is better than offline wallets.
You have never used an offline wallet but cannot give any reasons to use an online wallet, yet that's what you use all the time? I'm confused
The only reason why people might be willing to use a centralized online wallet is the lack of knowledge to set up and run their own desktop or hardware wallet, and the accessibility. There are people who don't cary around any electronics with them and might want to log in and send funds from work, school and so on.
Some people get lured by referral bonuses and such but that's really not worth it.