I'm going to make a big mistake and try to answer based on my memory of the
subject, Don.

Viscosity, low temperature stability,  and volatility are some of the
problems with these esters
which blending with a little petroleum diesel helps to iron out.

Biodiesel is based upon natural fatty oils which can range from 14 to about
18 carbons in chain length.
They come from the plant or animal sources as a triester with glycerine, and
are rather viscous.  They can be
refined including such processes as splitting the triglyceride ester,
releasing the glycerine which is removed
and forming a new monoester.

The new materials are usually  methyl, ethyl, or isopropyl esters of the
same fatty acids.  They are chemically
a bit different from normal petroleum hydrocarbons.

The chemically unsaturated esters (oleic, for example) are more likely to
stay liquid than a saturated ester
like stearic.  Both oleic and stearic have 18 carbons and are the principal
products of many plant and animal
oils.  The stearic ester would be more likely to form waxy precipitates in
cold weather, and the petrodiesel
would also help prevent this from happening.  If you dont distill the
natural oils (which you dont want to do
for reasons of cost) you would invariably have a mix of oil types, some
desirable, others not.

It's not needed. It's there because at this point, biodiesel is more
expensive than petro diesel.

Actually, the gasoline in E85 is primarily added for safety reasons:
Ethanol burns with invisible flames in the daylight; gasoline colors the
flames bright yellow-orange.

There are a couple of reasons. One is that biodiesel doesn't like to
ignite in a cold engine very well. Once the temperature drops down below
60 degrees or so it doesn't ignite very well. Another reason for the
blend is that biodiesel turns into a solid mass (think crisco) at
temperatures below 40 degrees unless you heat it up, since very few
vehicles have heated fuel tanks they add regular diesel to keep it
flowing in the cold. I have also been told by a few diesel mechanics
that the biodiesel is hard on the injectors so the added diesel acts as
a buffer for the injectors.