I only used a diluent, or solvent, for three specific reasons. One, if the material to be diluted was very strong and proved impossible to be used at a 100.0% concentration; for example I would only ever use Thiomenthone as a 0.1% solution, or less. This chemical is very strong and smells unpleasantly of cat's pee, but in dilution smells of Blackcurrent Buds. Two, if the undiluted material was horribly viscous, and would be difficult to compound with undiluted. For example Benzoin and Oakmoss Absolute are very hard and difficult to measure accurately. They would be diluted with a suitable solvent (usually to 50.0%) to make handling easier. Three, to dilute the whole fragrance to cheapen it, or for the solvent to act as a carrier for some specific application (e.g. Candle fragrance, or Reed Diffuser fragrance). I rarely used any other diluted materials, and cannot think of a time when I would dilute Essential Oils. Once the fragrance is complete, the whole thing may be diluted into a suitable solvent (e.g. Alcohol, if making an EdT).
What solvent you use depends on what you want to dilute, and what the end product is going to be. I rarely used fixed oils as they cause so many problems with solubility, and If I had to use them (a customer requiring an all natural fragrance, for example) the oil of choice was Castor Oil as this caused the fewest problems. If I didn't need to worry about the entirely fatuous reasoning for all natural fragrances, the solvents I would use were Dipropylene Glycol (DPG), isoPropyl Myristate (IPM), Dioctyl Adipate, or a mixture of of solvents called Downol. The Oakmoss and Benzoin, and other such sticky hard resins were usually pre-diluted in either Benzyl Benzoate or Benzyl Alcohol.