Can someone explain the difference between the various types of tram point
signals that have been proliferating in recent years?

The oldest style seem to be the bar type, with a horizontal, angled, and
vertical bar, which I assume indicate the lay of the points. They're at
almost every junction but despite clearly indicating which way the route is
set, the driver must still perform a compulsory stop before the points to
double check, which strikes me as a bit unnecessary. I was under the
impression that the horizontal aspect indicates that the points aren't in a
defined state, which should be adequate for those situations when the
points haven't functioned as desired by the driver. Eliminating this rule
with some sort of positive route locking would speed up movements through
busy junctions; to the best of my knowledge this rule doesn't exist in
European (especially German) systems, but I might be wrong.

St Vincent's Plaza has a different style that has an arrow indication and a
diamond above that. How does this style differ from the bar type? I asked a
driver and they said it was because the E-class are too long and foul the
blades, preventing the compulsory stop that would otherwise be required.

The newest sort are these double-bar signals, which don't appear to
indicate points at all. So far I've only seen them at the Nicholson St ×
Blyth St and Toorak Rd × Glenferrie Rd termini, and they appear to be
interlocked with the points on the terminating roads.

There's also a fourth type, which I haven't been able to get a photo of
yet, that uses amber instead of white LEDs, and shows either a * < * or
* | * indication. These are only in three locations that I'm aware of:
outside the NGV where Route 1 branches off, at Domain where the 8 formerly
branched off, and at St Vincent's Plaza coming from Victoria St. I assume
this style gives a preliminary indication as to the lay of the points, but
I don't know for sure.

Why are there so many ways of communicating essentially the same
information?

Robbie