In the past I have used it to make fsync and similar a no-op. ( This allows you to explicitly turn off data-security for certain processes, in order to either get performance, or to prevent them from affecting the rest of your system)

LD_PRELOAD loads symbols from dynamic libraries before "real" dynamic linking happens. This can be used to inject or replace certain symbols with your own definitions.

The classic LD_PRELOAD trick is to replace malloc and free with your own functions that contain some kind of instrumentation for debugging/profiling purposes.

Tinkering with LD_PRELOAD (and LD_LIBRARY_PATH) may have unintended consequences, so it's not a good idea to use them in your environment permanently.

I've abused it in the past when an application depended on a library that was not installed under the system path.

I'd agree with the people who warn against using it when not absolutely necessary.