Why use infantry AT weapons?

[Blackcloud6]

I was also under the impression you had to decide to roll for AT OR do normal assault. What you guys are saying is that they can roll for AT and if they don't get it they can still normal assault?  If so I'd agree that is way more useful. I also questioned the use of Inf AT weapons in assault.

Third Edition allowed you to do both, Fourth Edition says you have to choose.