Because if you want a nice heavy current into a battery, you are going to have to boost that 17 v supply to more like 170 to get tens of amps into a sulphated battery.
Caps would work too, but then you have to switch the full current into the battery, while holding off a high voltage. using a coil lets the current build slowly at your voltage of choice and dump that current through a cheap diode quickly. I've seem some circuits that use SCRs for this as well.
