Hardware-less solution

I’m new at telephony systems and have been researching into a web-based PBX solution and the more I research the more I realize Asterisk must be the solution. However, I’m still confused as to the use/necessity of hardware.

As I understand it, the main hardware that might be needed would be the Digium cards? But I’m still not sure why I would need them…? What advantages do I have over a software-only solution if I use the hardware?
Cannot software alone do the trick?

I need the ability to automate several traditional phone calls from a website.