> I have no clue why maples would produce nectar.  

>> Because they are either "mostly" or "partly" insect pollinated.

>>> Species do not simply do this "because of" that.

Cause and effect is always testable.  "Because" is an expression of cause
and effect.

The greatest diversification of insects began about 120 million years before
the emergence of angiosperms.  Insects (mostly beetles back then) were very
abundant, so plants that evolved flowers attracted insects, and were thus
pollinated more often. So solely because of their flowers, the flowering
plants got pollinated and proliferated. The flowers most attractive to
insects got the most attention, and hence pollination. 

Solely because of the more attractive flowers.
The ones producing the greater nectar reward are more attractive to honey
bees.
 
So, those plants that produce nectar today do so solely because their
ancestors did so, and got pollinated.
So they do, strictly speaking, produce nectar "because" that same nectar
proved to attract insects in the past - Q.E.D. for cause and effect.
Yes, some flowers do not seem to attract an insect (any longer), so one is
prompted to wonder if the insect subspecies attracted went extinct, or if
that flower was an abject "failure" in terms of attractiveness from the
get-go.

Not to confuse cause and effect with intentional acts, of course.  
(Except for Morning Glories - they clearly have the overt intention of
taking of the world, one fence, one garden at a time.)  

In chronological order, one can map a broad summary of the history of
evolution of both bees and plants as follows:

The Time Before Land

The Time Before Bees

The Time Before Thyme

The Time For Bees On Land

The Time For Thyme

The Bees Land On Thyme

The Land Before Daylight Savings Time

Now

(There never was a "Land Before Time", as space for land to exist in does
not exist without time also passing.)

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