Recipe conversion question/late addition

I am converting a Belgian dubbel recipe from partial mash to all grain. While most of the ingredients are introduced at the start of the boil, this recipe says:

Add LME and turbinado sugar at 15 min.

Since I have converted the LME to grains and will go in with everything else at the start, I am not sure how this works. I am kind of guessing that this will become:

Add turbinado sugar at 15 min.

Can someone point me in the right direction about the effect I would be losing here but only adding sugar so late in the boil. And what the general purpose of adding sugars later in the boil is. Thanks!

The late addition of LME and sugar was probably done to avoid lowering hop utilization with the higher gravity those ingredients would have caused if introduced earlier in the boil. You might want to take that effect into consideration when coming up with an AG version...

Cheers!

Yeah, add the sugar with 15 minutes left in the boil. I haven't used sugar yet, but when I add Belgian candi sugar, it is added with about 15 minutes left in the boil.