Why are the final copies of thesis dissertations often required to be double spaced? From http://thewallaceline.blogspot.com/2006/11/double-line-spacing-in-dissertations.html:


  It is standard practice to require dissertations to be double-spaced. In the age of word processors, one wonders why this practice persists. This must surely be a legacy from the days when dissertations were typewritten and students were allowed to make corrections in situ rather than re-print. Word processors have eliminated that problem. Double-spacing might also be suitable for drafts which require annotation for editing, but final copies of dissertations are not used in this way and modern techniques such as commenting do this job.


How did this tradition get started? The ability to make corrections on the printed thesis in the days of typewriters sounds like a pretty good guess.
My understanding was that this is for the convenience of examiners, who may wish to annotate a printed copy while reading the thesis, for reference during the defence. In the UK, at least, where candidates may be expected to make corrections to their thesis after the defence, this is quite common. Annotation is much easier if there is plenty of whitespace on the page.