There are two uses for guard rings. One is to make the
lowest practical Rb on the parasitic transistor structure.
Some foundries only require that you respect a minimum
"distance to tap" but a guardring can improve this worst
case extrinsic base resistance by a couple of decades.
In flows where the looser construction style is allowed
in the core you may still see guardrings called out in the
I/O or ESD device rules.
The other use is to backstop any field or well surface
inversion, which may occur in lighter doped or lousier-
field-oxide processes, or in the presence of ionizing
radiation.