How To Do It...
If one has never processed film or printed film, a sensitometer can give some guidance, but perhaps not much more than tables or experince of others.
The main utility is to answer the question of how long a new film developer combo needs to be developed WHEN ONE HAS ALREADY PROCESSED A FAVORITE FILM IN A MANNER THAT GIVES EXCELLENT PRINTS.
One needs to expose the KNOWN film with the sensitometer, and process it in a KNOWN manner. The results are read with the densitometer and plotted. Then, using some indicator of slope or gamma (many ways to do it, just be consistent) find the slope or gamma for the known film.
Then expose (with the sensitometer) and process the unknown film and measure its slope or gamma. Then alter one's development time to adjust the slope or gamma to match that of the known film. More development to increase slope/gamma and less development time to decrease slope/gamma.
The utility of these methods will become more important as we photographers are forced into unknown film/developer combinations by market availability of products.
There are many, many other uses of sensitometers and densitometes in fine art photography, but the above mentioned testing sequence is the main reason why I think almost everyone would probably benefit from a sensitometer.
As an analogy, if one is taking pictures in the bright sun without a light meter and then one wishes to photograph inside a dark building, one does not NEED a light meter to get good exposures, but having one can really help. In a similar manner, when moving into an unknown film developer combination, one can get by without a sensitometer, but having one can really help.