Add mobility without foresaking security: Even though SharePoint is an online cloud based SaaS product, you don't need to worry about the security with the ability to keep your information behind a VPN, without having to create additional infrastructure for authentication.
Some organizations choose a hybrid approach due to highly customized security or workflow needs. This can be a great way to push extra storage to the cloud or utilize the full strength of SharePoint on-prem with a lowered overall cost and flexibility that you need.
Outsourcing everyday IT tasks:
SharePoint Online takes over some of the backup and recovery duties that your IT team may get bogged down with in a strict on-prem environment. Once you've signed the SLA, these problems become Microsoft's - not yours.
Cost control: Many organizations, big or small, want to bring collaboration solutions and functions to a wider range of their workforce while reducing overall cost. Since this is a license-based tool, you only pay for the users and level of licenses that really need it. Give some users a limited license while giving others all the bells and whistles they need.