Taking an office changes what a person is allowed to do. An oath marks that change by naming obligations they accept before others. It makes authority appear as an entrusted role with commitments, rather than simply a possession the successful candidate has acquired.
The US Senate oath has changed with political circumstances. An oath is therefore not just timeless ceremony: what it asks someone to uphold expresses a particular understanding of the office.
A promise other people can refer to
Imagine a newly elected association officer publicly committing to follow its constitution and handle shared resources faithfully. Members can later refer to that commitment when a decision appears to serve the officer's private interests. The ceremony gives the obligation a memorable public form.
The words do not enforce themselves. A person can repeat them sincerely and still misunderstand a duty, or repeat them cynically and violate it. Records, oversight, and consequences determine what happens when conduct diverges from the promise.
The oath's distinctive work is expressive and relational. It tells the officeholder and the audience what kind of authority is being accepted, and whose rules or purposes it is meant to serve. That shared declaration can matter without being mistaken for proof that the declaration will always be honored.