A - Thanks for the question. A bit more of the history should help.
The oldest Biblical manuscripts do not include what is called the doxology of the Lord's Prayer - that is, the words "for thine is the kingdom, the power, and the glory, now and forever." It is thought to have been inserted into later Biblical manuscripts (only in the Gospel of Matthew) somewhere in the Eastern Churches early on, so it is an ancient Christian prayer, though it is not Biblical. It was most likely used in early Christian liturgies and a form of it is present in the Didache, an important Christian document written around 90 AD. Chapter 8 says (emphasis added):
But let not your fasts be with the hypocrites; Matthew 6:16 for they fast on the second and fifth day of the week; but fast on the fourth day and the Preparation (Friday). Neither pray as the hypocrites; but as the Lord commanded in His Gospel, thus pray: Our Father who art in heaven, hallowed be Your name. Your kingdom come. Your will be done, as in heaven, so on earth. Give us today our daily (needful) bread, and forgive us our debt as we also forgive our debtors. And bring us not into temptation, but deliver us from the evil one (or, evil); for Yours is the power and the glory for ever. Thrice in the day thus pray.There are several different versions of the doxology that are found in early Christian writings, but it seems to have been standardized later on.
There is nothing wrong with praying the prayer. Thus, we use it still in our Mass, as we do many other ancient Christian prayers.