https://www.warhammer-community.com/2017/05/08/17794gw-homepage-post-4/
So this is an interesting rule. It means that Dreadnoughts get wounded by bolters on 5+.
In close range, double-tapping bolters will hit on a 3+, wound on a 5+ and be ignored on a 3+.. That means that a unit of 9 marines, shooting 18 times, will get:
12 hits.
4 wounds.
1.33 after saves.
But even MORE important, you have Tau Fire Warriors, which now wound Knights on a 5+.
You won't be causing 20 wounds to a Knight, but even the biggest Knight has to accept a little battle damage when getting in close with Tau.
A couple of units of Fire Warriors in a transport, dropped off in close range will knock off 4 wounds. Not a bad way to finish off that wounded Knight.