I am a sales consultant for a builder. Its margin in the deal just like anything else. Make an offer Sunday night or at the end of the month. I have held turning in aggressive deals to get them approved for that reason. Not every builder works exactly the same, but lots do.
I've had buyers negotiate better deals because we didn't have to pay an agent on it.
I've had realtors negotiate better deals than their clients would have without them, because they ran comps, and didn't care about holding on to their bonuses to get their client more money or were mortgage savvy or whatever.
I like working with agents most of the time and I have btsas3 right now. I would never tell a prospect in front of me that they "get a better deal" without a realtor, but the reality is that when I submit a deal that is outside of my standard incentive parameters, the first question is "do they have a realtor" because that's a significant $ amount in the bottom line.
If you are building from scratch, dont like haggling, or just feel uncomfortable with the process, a realtor can absolutely help. If you have any questions, I'm happy to answer any of them.