What's it for?
Email service providers (e.g. Hotmail, Gmail) have various ways to ensure that an email you receive is from the organisation it claims to be from.
One of the things that they sometimes do is show additional information about the sender of an email. This helps users of their email service avoid potential scams and gives a user confidence in authenticity of messages they receive.
Different ESPs use different rules, so there is no hard and fast guide as to when this extra information shows.
If an ESP isn't 100% sure your message is trustworthy, it may show this additional information about you as a sender to help the recipient determine who you actually are.
In these cases, some recipients may see a "sent on behalf of" message appear next to your sender name.
This header doesn't hurt your email deliverability, it's just an informative message to recipients at ESPs such as Outlook and Gmail.