A curve parameter domain can be also from negative to positive or it can start not from zero.
It is not a bad practice to always reparametrize, but if you do so, remember to reparametrize the curve once for all at it source or everywhere it is used.
Parameter is unrelated to length, more like it is related to control points locations.
See how much length is covered at start vs the end, even tough the slider is changing almost constantly:
