Probably most pros would be using densitometers to read control strips for tracking the process control of whatever they're running--E6, b&w, C41, RA4 etc. I use one where I work to measure E6 control strips and occasionally do Kodak b&w control strips as well. You'd use these in setting up a deeptank line or a processor more or less--for the b&w, you measure the "contrast index" which is calculated off a couple of densities on the control strip and corresponds to how well a given time will produce a neg that prints on grade 2 paper. Once you establish a standard dev time for a given CI, you can then figure out higher or lower CIs and additional dev. times. In the end, if your process remains consistent, you have a pretty good idea of what times, i.e. batch times, to set up processing runs if you have to run alot of different types of film....you also use the control strips to monitor your process & track replenishment, fixer activity etc. For b&w you can pretty much do this without a densitometer--it's pretty easy to figure out film speed by doing ringaround tests with gray cards, but for any color work you would definitely need one at a certain point. In E6, the strips are used to set up the first dev. time, which sets film speed. You also use them to monitor other steps that ulitmately effect color balance etc. FWIW, the densities you read are plotted on a graph--chart--against "aims". You have a certain amount of tolerance to work within on the upper & lower sides of the aim. For b&w the aims are set, but for color they vary by the emulsion batches of the control strips. The aim for this is reached by initially reading the "reference strip" which comes with each box of control strips. The point in the end, is to aim for a standardized process, and you really need a densitometer for this sort of work, although what you're doing is reading control strips really, checking them against the manufacturer's standards.