A very simple and short answer is: because no one knows everything.
Just as most people does in real life for a variety of important matters, getting a second opinion it might result of huge benefit when it comes to your ERP.
You got that estimate and there's something in it that seems out of place. It is not necessarily that you don't trust your implementation partner but that you want get a different set of eyes to validate your planned investment, it is not about spending less or implementing faster, it is about "peace of mind" that comes with confirmation. Perhaps it is just the the project is so big or critical that you need to back up your decision.
There are 2 golden rules for second opinions:
Do not go cheap. The reason you are needing a second opinion it is because you are not an expert or you don't have enough information to make a decision. If this would be your health or something else of value then you know it is not right nor enough to jump onto the internet and "learn on the fly" or even worst asking Siri or Alexa what they "think".
Be open to share. The only way to get value out of a second opinion is by sharing all bits and pieces with the SME for him to have the best possible understanding and enable him to come up with an accurate diagnostic. Second opinions are not about challenging somebody to guess the same that you know from others.
We can help you get that second opinion for you ERP project, feel free to contact us for more information.