The purpose - whether functional or purely symbolic - of the kampilan spike has been lost over time. Some claim it's for blocking or trapping an opponent's blade, others claim it's for stopping overpenetration on a thrust. I don't believe either (in principle, stopping overpenetration would make sense, but a kampilan isn't a thrusting sword). Decorative and/or talismanic.
It won't do anything significant when cutting with a kampilan. Some have it, and some don't. Clearly not essential. (However, at least some of the spike-less kampilans out there originally had, and it was broken off.
A photo with my super-spiky kampilan:
(I have another without a spike.)