Question
Would it be helpful to use creosote branches and leaves as a mulch or organic amendment to hold moisture?

Answer
From the phrasing of your question, I am not sure of what you are speaking of.  If you are speaking of the _Larrea tridentata_ plant commonly called the creosote bush, then you can use such trimming, etc. for mulch.

There are two categories of creosote: wood creosote and coal tar creosote. If you are talking of wood creosote, which is now rarely found, then there is little harm done in using such products as mulch.  Since wood creosote historically was used as a cough treatment and laxative due to the high amount of plant phenolics, it is very, very unlikely that you have access to wood creosote.

The coal tar creosote is NOT an acceptable mulch as chemicals in those products are carcinogenic.  In some parts of the US, you may find railroad ties or utility poles that have been treated with coal tar creosote; these are not acceptable to use for garden mulch because of the exposure to carcinogens contained in them.  In many jurisdictions it is currently illegal to sell or use these products.