A guardring serves as a tap to the region of same species
that it's in. But a tap is not a guardring. Tap is point access
as minimum (could, or not, be bigger like bar or ring).
Think of fencepost and fence.
Completely enclosing a device can eliminate surface
conduction (field leakage), provide a much lower
access resistance to the region of the device you
care about tying down, and more capably **** up
wandering minority carriers (recombination @ high
doping, pinch of epi / well cross section, depletion
sweep for counter-doped guard rings). A lonely tap,
you get only the carriers that diffuse to within its
"sphere of influence" (or half-sphere).