The purpose may be that because alignments are broad and oaths are narrow, but at the same time oaths do not strictly belong to one alignment only, combining them in different ways means variety, without necessarily be completely divergent.
For example a "Paladin of Nature" perhaps is usually Neutral, but how about a good Paladin of Nature and an evil Paladin of Nature, maybe the first personally values life even tho he's taken an oath to protect nature, so he's going to have interesting issues when killing someone is the best way to protect nature but perhaps not the only way; the evil counterpart may have issues when protecting nature strongly contrast his own benefit.
Another example could be a Paladin of Justice. The most natural alignment could be LG or LN, however you can come up with interesting characters that are instead LE, N, NG, so not necessarily completely opposite like CE.
Anyway alignment is going to be optional. Oaths are going to be more specific, which means that there is way more than 9 possible oaths that you can come up with (in contrast with 9 classic alignments), but at the same time each oath should not be restricted to one of those 9 only.
In conclusion the way I see it, if you use also alignment in your game, you're going to apply that to the character's personal moral feelings and belief, while the oath is like a vow taken. Most characters obviously will take vows that match their morals, but some may diverge for a variety of reasons, such as having compelled by circumstances (out of a promise given, or as part of a deal) or simply because their morals changed during their lives.