Never put all eggs in one backet.That`s why.
I still afraid that the online wallets i use might be hacked or run away with my bitcoins.
Yes, that is one of the reason. Another reason is, may bitcoin users are trading in different exchanges. So it is obvious that they will be using multiple online wallets. Others use the offline wallet for security, and dividing their bitcoin holdings in different light wallets like electrum and multibits. Some software is not compatible with other devices so they need to download the needed application for it too.