When Dagney Taggart spent time in the Valley, she asked to be allowed to attend John Galt's lectures on physics. He refused on the grounds that he didn't want to give her information or ideas that could be used to help people on the outside, and that all of his students had a practical need for it (implication: she didn't). Some of his students included (among others) Dwight Sanders (formerly an aircraft manufacturer), Ted Nielsen (formerly a motor manufacturer), Quentin Daniels (professional physicist), and Owen Kellogg (formerly a manager for Taggert Transcontinental).
Most of those make perfect sense - physics has obvious applications to manufacturing aircraft and motors. I don't understand what Owen Kellogg would do with the information, though; also, it seems like if it was useful for Owen Kellogg, it would be equally useful for Dagney (which John Galt strongly implies that it's not).
Not wanting to give her information that could be helpful to outsiders before she had joined them makes perfect sense, but why the practicality objection? Why was Owen Kellogg on the list of people who had a practical use for the information but she wasn't?