If i start to rent my home for the first time, why do I have to enter purchase information?

If you are renting part of your home, then that part is now considered to be business property.  As business property, it must be depreciated.  TurboTax needs the purchase information and other data in order to calculate the depreciation of the business portion of your home.  

You do have to be aware that the property tax that you paid will now be split between the portion of your home that you use personally (Schedule A) and the portion of your home that is business property (Schedule E).  You may be required to enter the property tax more than once, but between the two forms (Schedule A and Schedule E) you should not be claiming more than the total amount you paid for the year.