Matthew my friend this is actually a great question - thanks for asking it!
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Well you already know that finish protects the guitar from the things we humans leave behind. Finish, at least some finishes will also protect the guitar from an occasional bump, mild forms of scratches etc. And finish can be cleaned much more effectively and easier than bare wood can be cleaned.
Finishes do impact tone more or less depending on the finish and thickness. But..... the tone that many of us have become accustomed to is the tone of a guitar with a factory finish. And factories have traditionally favored finishes that are better at withstanding abuse.
IMHO nothing gets you closer to the sound of raw wood than a French polish finish but the trade off is that FP does not provide the protection of nitro or cat poly.
So, in short, guitars need to be finished so that they will be more practical in terms of the things that guitars will encounter that may damage them. If you ever want to know to what degree the finish impacts the tone of a guitar build two alike and French Polish one and use nitro or something else tougher on the other. I did this with 2 L-OOs and the difference is like night and day. But they are different, one is not better, just different.
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