The boost grab is pretty much a better version of the dash grab. They have the same hitbox and the same amount of lag afterwards. If for some reason you want to run and grab but don't want the boost you should JC grab.
As far as JC grab vs boost grab, JC grab has less lag if you miss and a better hitbox. The advantage of boost grab is you can get to your target faster when you're running, because of the boost.
Boost grab is good for techchasing (the most important use), chaingrabbing in some odd situations (Sheik dittos at 0 I think), and possibly for mindgames, like when you are running at your opponent looking like you might dash attack but boost grab instead. For other uses JC grab is the way to go.