In theory, penis extenders should cause new tissue growth in the penis.
However, penis extension devices require a significant commitment to wearing them (3,000 hours+), and most of them are pretty uncomfortable.
A penis extender will increase both the flaccid and erect penis length.
While the theory supporting penis extension has some validity, in practice, a significant permanent gain in length is seldom achieved.
Most users wear the penis extension device for a while (maybe a month or two), before concluding that the insignificant gains in length (if any), are not worth the inconvenience of wearing the device.