Dende creates multiple sets of Dragon Balls, weaker than the originals, hoping that the young warriors will benefit from competing over them. The presence of multiple sets allows for warriors of diverse skill levels to have a realistic hope of gathering all seven. Afraid of them being exploited, Dende limits their power, restricting the wishes that can be made.
One of Dende's sets of Dragon Balls (though weaker than the others) created in this year, ends up in Toki Toki City (circa Age 850) and are used by Future Trunks and Chronoa during their battles with the Time Breakers and Demigra in Age 850. This set continues to be used by the Time Patrol in Conton City in Age 852. Due to the nature of Toki Toki City/Conton City being connected to different eras, these Dragon Balls can be used endlessly and do not have a period of dormancy (during which the Dragon Balls are inert).