I think the title is self explanatory: why wouldn't either one or the other be enough?
After removing the bolt, you still need a specific tool to pull off the crank arm from the spindle, because the crank's teeth are holding to the spindle so strong, that you cannot just remove it by hand (so it is even much less probable that the arm falls off by itself, even without the bolt). What is the bolt for then?
Alternatively, the bolt already presses the crank arm to the spindle very strongly, why do we need them to be engaged with the extra teeth then? And due to this also a specific tool to remove the crank arm is now required, whereas with the bolt only a standard tool (either a hex wrench or a socket wrench) would be sufficient. (What's even worse, you risk ruining the thread of the crank arm, if you are not careful enough with this tool.)
To be clear: I know there must be a good reason to have both, I just want to better understand what this good reason is.