The most obvious reason that comes to mind when someone talks about blood flow is having stamina when running or doing some sort of physical exercise. Running in specific requires you to have a strong heart to be able to pump blood though out your entire body. One often forgotten and not so talked about reason to have good blood flow is during sex. How long you go pleasuring your partner has a direct connection to your physical condition.
Lots of people take shortcuts and end up taking products that they think might help their under performance issue. That is why there are some many reviews of vigrx out there. Most of them are made by people who this they have beat their issue with a shortcut. It is not worth taking shortcuts. There might be a larger problem beneath it all. Its best to go to talk to a doctor before purchasing any sort of enhancement pills. They can be a lot more expensive in the end than you think. Your health is not something to play with. You only have one body. Treat it right.