In the beginning of the Vocaloid craze, voicebanks were much more limited. This was true in the issue of varity of the voicebanks being offered. Until Gakupo, the only solid male voicebank was Kaito and LE♂N. While its true that producers had Len, he was of a young male type.
So to get as much as one could out of the software itself, you had to manipulate the voices you had, especially since there was a lack of male voicebanks at a time. The act of gender bending a voicebank was a demostration of a voicebank being manipulated from female to male or male to female. This opened up the creativity and was all part of the reason why there were so many fanmades early on. This was also why a sudden interest in English Vocaloids opened up when the Producers realised there were more voicebanks then they thought, although not many could actually use them sadly.
This process still is around, though there is less of a call for it. However some very famous genderbends like Kaiko are still used today.