Nautical miles are used in navigation because one nautical mile equals one minute of latitude when you're traveling North/South, making it a very useful unit of measurement on oceans. Planes use it for the same reason. A knot is simply a speed of one nautical mile per hour, so it's a useful unit for speed when you're using nautical miles for distance.

So I wouldn't call these two specific examples relics, but I agree with you on uniforms & ranks.

Specifically, the nautical mile thing means that any chart with coordinates on it also has a scale in miles built-in.