The wallets that truly blow my mind are the handful that still don't give you private keys such as Freewallet, let alone Coinbase and Xapo of course. Why anyone would go anywhere near them is an absolute mystery.
Pretty easy. These platforms offer people instant and free user to user transactions.
Also, nowadays there are lots of faucets and other reward sites forcing people to use these platforms in order to receive very small payments in Bitcoin and other currencies.
Most people just accumulate smaller payments on these platforms and later on withdraw them to their own wallet or an exchange for trading purposes.