That it increases the uptake of ED drugs.  It's like taking a stronger dose.
.
Another way to increase the effective dose is to let the ED dissolve under your tongue.  That can be pretty bitter for viagra, but it also gets into the system in minutes.

For tadalafil (Cialis) at least, lime juice has a similar but smaller effect of increasing uptake.  But orange and lemon do not.  
.
Just in case you want the effect but hate grapefruit.