1. A
menace
2. The theory and practice of time travel is
incomprehensible to anyone who hasn't spent at least four lifetimes studying advanced hypermathematics, and since it was impossible to do this before time travel was invented, there is a certain amount of confusion as to how the idea was arrived at in the first place.
3. An easy way to make money by copyrighting things, then
suing the companies who originally copyrighted the thing in the first place.
The Encyclopedia Galactica copied many of its excerpts from the back of
cereal boxes adding footnotes so the companies couldn't sue them. It is ironic that due to time travel the Encyclopedia Galactica went back into time and wrote the passages before the cereal companies, and then went on to sue them for copyright infringement.
See also: The Hitchhiker's Guide to the Galaxy,
Life the Universe and Everything,
The Restaurant at the End of the Universe, Douglas Adams, and Encyclopedia Galactica