# Purpose of construct With[{var=myComputation[…]}, var/;True]? [duplicate]

Could someone please give an illustrative example showing the purpose of construct of form With[{var=myComputation[...]}, var/;True]? Also, I wonder what kind of reference material are available for getting familiar with programming idioms for Mathematica. I mean besides the basic syntax that one can learn and re-learn from standard documentation, there are repeated patterns that programmers use. Somewhat similar to design patterns and "proramming pearls" for Java.