Why you need a sensitometer
The need for a sensitometer is similar to the need for an exposure meter. One does not really NEED it but it can help out. With experience its use becomes less important, but when faced with the unnusual or out of the ordinary, it is a neccessity.
People learned photography years ago with instruction on how to use a light meter. Sensitometers were not deemed neccessary (in my opinion) simply because they were so EXPENSIVE. A current unit from Agfa cost $24,000. They are so expensive because they can be calibrated for ISO standard determination. Unless one is running an ISO standards laboratory, this is unneccessary.
THE MAIN REASON THE FINE ART PHOTOGRAPHER NEEDS A SENSITOMETER IS TO DETERMINE A STARTING POINT FOR DEVELOPMENT TIME.
If you use the same film and developer combination all the time you DON'T need a sensitometer. Likewise if you shoot in bright sun all the time you DON'T need a light meter. I was like this for many years, however, the film and developer that I like may soon be extinct.
UNKNOWN FILM/DEVELOPER COMBINATIONS ARE THE MAIN INDICATION FOR SENSITOMETER USE
For generations one had to guess at a starting point based on mfgs tables or the experience of others or trial and error.
Use of a sensitometer (and densitometer) puts some order into the process and allows one to zero in on a development time by comparing the slope of the new film with the slope of a known good film/developer combination.