For assault to occur, your infantry needs to be in READY status, rather than PINNED.
Whether you succeed or fail, your infantry will end up taking some suppression. However, even if you fail, the tank unit will also suffer suppression, possibly leading to RETREAT.
1 more thing, if you do not succeed in 1st time assault, you may rally till READY and try once more. I've ever succeeded on my 2nd try.
Assaulting a tank will allow you to hit at the weaker TOP armour, which in most light tanks is 1. And most grenades have a PENETRATION of at least 1, meaning successful destruction of the tank.
The only way to clear bunkers is also assaulting.
I think this post provides further information on this topic itself.
http://forum.shrapnelgames.com/showthread.php?t=47111