The basic advice is: Do not use it unless you have a specific reason that absolutely requires it.

In general, you should not fix absolute addresses in your software by any means (_at_ or similar, Linker directives, etc) unless you have a specific reason that absolutely requires it - the tools are generally best left to handle this.

The next question, then, is "what specific reason would absolutely require it?".

At the moment, I can think of only two:

1. Memory-mapped IO - in which case the address will (generally) be fixed by the external hardware, so the software tools cannot know about it unless you tell them explicitly;

2. If you require a version number or similar at a well-known location in your ROM; eg, so that it can easily be found and read using a PROM programmer.

One other is in a multiprocessor system where processors communicate via shared RAM - but this could be considered just a special case of 1 above...