Office Expenses are recorded in Expenses section, because they will be for the use of office and not to sell.
Example - You buy books and pens for the office use and not to sell them.
Purchase is you buying the products from the vendor for the purpose to resell them again, and this will get added to the Inventory as well.
Example: If you are a milk vendor you will buy milk at 35rs and you resell it at 40rs.