It's tempting to assume that flower constancy evolved for the benefit of plants. The following quote demonstrates this conjecture:

The ecological meaning of flower constancy is easy to
see for the plants as well as for their visitors.
Constancy gives the flowers their only chance to be
pollinated with a sufficient amount of conspecific
pollen ... Kugler 1943

However, more recent studies have come to the opposite conclusion:

There is no good evidence that flowers have evolved
to favor constancy, or that constancy contributes to
the development of complete or nearly complete reproductive
isolation. However, there is good evidence *against* the
notion that pollinator constancy is involved in speciation or 
maintenance of plant species integrity.

Chittka, L., Thomson, J. D., & Waser, N. M. (1999). Flower constancy, insect psychology, and plant evolution. Naturwissenschaften, 86(8), 361-377.
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