You are correct that many photographers do not use a densitometer in establishing the EI of the film that they are using or the development time of the film. In fact many do achieve remarkable results.

To answer your question regarding the practical purpose of a densitometer. The densitometer will more accurately determine the actual EI of the film and the contrast range of the negative.

I have done it both ways. With TriX I established through the use of a densitometer the actual EI of the film was 160 rather then 320 as advertised. With Bergger BPF I arbitrarily established 64 rather then the 200 as advertised. I exposed two identical scenes with both TriX and Begger. I placed an important low value at a Zone II luminance. When the film was developed the TriX is properly placed. The Bergger Zone II value is too high and shows more detail then desired. So I will either need to arbitrarily set a higher EI for Bergger or actually do the testing with a densitometer.

I hope that this answers your question. Good luck.