subwoofers were not always (and some are still not) huge monsters that requires jigawatts of power..smaller sub or even some old school subs require less power and hence the amps serve that purpose.
so why choose a mono? well consider if you have a sub that is svc (to keep things simple for now) and it requires 400 watts rms. u can find a 4 or two channel amp that gives you that much power on bridge bt then it will be very expensive. monos are used for subs because they are designed for higher power output and have appropriate LPF settings. most monos will not work with speakers or will only give low freq output.
another use of mono is that when you have dvc subs. the resulting impedance will be some thing other than the normal 4 ohms. most 4/2 channel amps are not stable at 2 ohms bridge. even less are stable at 1 ohm bridge. so to power more than one sub or to power a dvc sub, a mono amp is required which gives the right power at the proper impedance. be sure to check the stability of the amp at a particular impedance before buying it. some kenwood and pioneer monos are not stable beyond 2 ohms and will heat up or shut down at 1 ohm
