There is a cost associated with hiring a professional to measure your home, but this does not have to be a deal breaker. Do your homework. Find a vendor you have confidence in and work their quote into your budget.
Now you have no reason to ask "what is in home measurements?" or why you need them. There are many reasons why you hire a professional for in-home measurements. As a homeowner, you will not be liable for an incorrect measurement. A skilled professional is less likely to measure incorrectly, which saves you time and money.