@Sadmin Re: Using your finger...
Yes, using your finger can give you good enough numbers, if you know what you're doing and where you're taking your pulse. And its accurate enough.
But try doing that in the middle of a work out where you have to use your watch to try and get a six second count. Its not easy.
And its Medical grade EKG not EEG since we're looking at your heart and not your brain activity. ;-)
As to 'medical grade vs commercial grade' its a bit of a misnomer since you can purchase medical tech if you want to pay the price.