First, I think those are basically for insulation and nothing else,
right?

Would you use it in places where no insulation is necessary, but you
still want to put up a water tight barrier?

For example, I have a 3" PVC pipe that exits the house below the slab
through the block wall (the block wall extends below the slab to at
least 24" below, don't really know how deep it goes have not got to
the bottom of it).  Anyways I drilled a hole through the block wall
about 3"x3" for the pipe to pass.  The hole is uneven and it's through
a hollow block wall.  I don't need to insulate it since it's 18" below
my concrete slab, but I want to stop bugs, tree roots, rodents, or
rain runoffs from coming through it.  I patched it with concrete mix
on the outside of the hole.  But the entire hollowed space in between,
do I spray the cavity with this foam stuff?

Are there any fundamental differences between the Fireblock foam, door
and window foam, cracks and gaps foam and big gap foam?

There are many places in my block wall electricians have hollowed out
for recessed boxes.  I am thinking of filling those with foam.
However since the wall is made of hollowed concrete blocks, the space
behind the hole is big, sometimes it extends above and below to the
voids of the adjacent blocks.  Being in Miami I would like to insulate
best I can.  What do you recommend be the most effective way to seal
these holes?

These small cans are expensive at $4-$7 a can.  If I need larger
quantities, they don't sell them in garbage can sizes?

MC