Be aware that the builder's representative is just that - a person employed by the builder, who puts the builder's interests first, regardless of what they may tell you. Due to the high volume nature of new home sales, builder's agents are often paid less than a traditional commission (some are even on salary plus incentives) so it is common for builder's reps to use high pressure sales tactics with potential buyers.
There are a number of reasons to use your own buyer's agent who is experienced in new construction sales when purchasing a home from a builder, such as:
1) Negotiations -- a good buyer's agent that is familiar with the local builders will be able to guide you through the contract process and negotiate on things such as price, options, closing costs where possible. Most builder reps will not tell you that those things can be negotiable. Nor will they tell you that you have the right to select a lender and title company/closing agent of your choice.
2) Contracts -- a buyer's agent will insist that all terms verbalized by the builder's rep are put into writing so that they are enforceable later on. Your agent can also go through the builder's multi page contract with you and point out important terms that you may miss or not realize the significance of. They will also help you go through your final list of builder upgrades to make sure everything you want is there and that there are no extras you did not select. By the way, builders love upgrades as that is a significant source of profits.
3) Accountability -- having your own buyer's agent tends to make the builder more accountable. Mistakes happen during the building process, a buyer's agent will work with you to make sure mistakes are corrected, not ignored or covered up during the process. This is especially true when it comes to the Punch List. Builder rep's often prefer to ignore somewhat minor issues that can be major issues to the buyer.
4) Experience -- a buyer's agent who deals with new construction purchases will generally have better knowledge of the local builders reputations, their strengths and weaknesses. They can tell you what are normal building practices in the area and what are not.
5) Knowledge -- a buyer's agent should be able to run the numbers for you comparing a new construction home in a neighborhood or area vs one that is just a few years old. Sometimes the best deal for a buyer is NOT a brand new home, but rather one that is only a couple of years old that the owners have to sell due to circumstances.
Generally there is no additional cost to the buyer to have their own buyer's agent. Most builders price buyer's agent compensation into their pricing strategies and encourage buyer agents to bring customers as they recognize real estate agents as a significant source of business.
One last comment, if you want your buyer's agent to get paid for assisting you in your new construction home purchase, please make sure that when you visit the model homes or sales office for the first time that you bring your agent with you and register them as your agent of record with the builder. Otherwise, your agent may spend countless hours helping you through the process only to find out at closing that they are not getting paid.