starter with self contained battery (safer, no wires to snag in prop)
self contained glowplug battery (safer, no wires to snag in prop)
no panel or HEAVY battery and no wires

roughly the same costs
easier to use normally

offers the flexibility to start airplanes wherever they are
often start planes in taxiways, out near runway, wherever it's safe and more convenient than carrying them back

NiCds in starter pack easier to deal with than the heavier lead cell and pack has more voltage as well