IERenderer seems quite technical and I note that there are frequent versions, updates and fixes. Before I take the plunge I would like to know its purpose so I can decide whether it's worth my ongoing effort to keep it up to date.

IMHO, it is worth it. As I'm sure you know, HTML is continually evolving. Michael in der Wiesche does an incredible job maintaining IERenderer in response to new developments, user wishes, and rendering anomalies. He honors the Pegasus Mail legacy of security yet allows users to adjust security features to meet their needs and comfort level. His recent addition of a mechanism that notifies of new versions means you no longer need to keep an eye out for new version announcements.

IERenderer can be disabled with two clicks if you decide you want to return to BearHTML If you have that desire I hope you will post here first for advice. Perhaps we can help. If not, it will give Michael the opportunity to consider your needs or concerns.

Can you explain what IERenderer does and what its benefits are?

This may be more than you care to know but here goes...

Pegasus Mail was created well before HTML formatted email existed. As
such, it did not have HTML rendering capability. I believe that Martin
Ireland was the first to create an add-on that brought HTML rendering capability to Pegasus Mail. It is called Bearhtml and has been incorporated into Pegasus Mail for many years. Michael in der Wiesche then brought us IERenderer which utilizes the Internet Explorer HTML rendering engine; a better mouse trap so to
speak. It also has also been integrated into Pegasus Mail. Both exist (the explanation into why is another story). On a new installation, IERenderer is also installed and is enabled by default. Bearhtml takes over the HTML rendering duties when IERenderer is disabled or uninstalled.

IERenderer gets installed as a program named "Pegasus Mail HTML renderer <version>". It can be disabled within Pegasus Mail. This is useful when an HTML message renders oddly and you want to see what Bearhtml will produce or when you prefer Bearhtml but don't want to uninstall IERenderer. It can be uninstalled via Add/Remove Programs.

Finally, IERenderer uses components of Internet Explorer that will be in Windows operating systems for many years even though Internet Explorer is dead.