It was tacked on in the 50s (note how it kinda messes with the cadence) because of godless commies. There have been other little tweaks as well, some making it worse (e.g. "under god") and some better (abandoning the, herrrm, "Roman" salute). While always a bit weirdly nationalistic, the core of the modern pledge was written by a Christian Socialist and replaced one that was worse and more explicitly religious.
I guess it's not so weird that it exists and I assume many countries have some sort of boilerplate loyalty oath they can bust out as needed, but it's pretty messed up that it's mandatory for kids on a daily basis and fetishizes the flag as an object. I am quite fond of my country, and I think there are a lot of worse places to live, but our history is pretty messed up and our views on what exactly constitutes freedom and democracy are not unquestionable just because some clever provincial elites came up with a halfway workable system in the late 1700s. I suppose it's marginally better to build a national cult out of institutions and symbols than individuals, but it's still a terrible idea to treat patriotism like religious dogma.