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Other arguments in YMDC lately have centered on Overwatch and how/whether it negates any and all modifiers, special rules etc. that don't explicitly say "this works in Overwatch/does not allow and according to search this question has never been clarified via discussion here (or anywhere via google for 8th)

So for the sake of robust clarification on everyone's understanding of Overwatch and rule-supercedance by Codices/Indexes over the BRB,

The Markerlight table for T'au indicates Markerlights function "in that phase"

ie. Re-rolling hits of 1
Ignoring cover
Adding 1 to hit

And Overwatch says

"Each time a charge is declared against a unit, the target unit
can immediately fire Overwatch at the would-be attacker.
A target unit can potentially fire Overwatch several
times a turn, though it cannot fire if there are any enemy
models within 1" of it. Overwatch is resolved like a normal
shooting attack (albeit one resolved in the enemy’s Charge
phase) and uses all the normal rules except that a 6 is
always required for a successful hit roll, irrespective of the
firing model’s Ballistic Skill or any modifiers."

So, is there any benefit to having models fire markerlights in the Charge Phase?

My understanding would be that reroll 1's and ignore cover (assuming the charging unit started in cover) would apply but +1 to hit would not.

Yep, its not rocket science. The re-rolling '1's and negating cover can still apply, just not the +1 modifier.

But why would you fire markerlights during an overwatch ?

They only hit on a 6 and only allow you to rerolls 1s, if you hit one.

Given that you still need a 6 for all other weapons to hit it should be far more efficent to just shoot the "normal" weapons instead of the marker lights.

Except you are Close to firing your half or whole army during the overwatch, then there might be really a sense to allow the 1s reroll for the rest of your shooters