[Laser] Drivers?

[KD7JYK DM09]

Just getting back into playing with lasers...  It's been twenty years and 
the last ones I played with were tubes.

So, let's say I have a laser module that requires 3.0 volts to operate. 
Most lasers seems to state they  require a constant current rather than 
voltage, why?  I have a power supply that can provide 3.0 volts at more than 
enough amps.  Can't I just connect the module to it direct?  In what 
instances, and for what reasons, are "drivers" used?

Kurt