Sacrifices readability for performance. Sometimes that's a worthwhile compromise, but often times not. And if you plan on referencing $description more than once (which, most code would), then it wouldn't make much sense to run strip_tags each time.
I doubt it even sacrifices performance. The temporary results of strip_tags() still has to be allocated in order to be echo'd out -- so you're using the same amount of memory. $description will be de-allocated as soon as it goes out of scope.
Both versions of the code do exactly the same:
1. Take description from memory
2. Allocate string and put the stripped version there
3. Output the string (here you might save if you don't use output buffering since no copying to the buffer happens but just output)
4. Release the memory allocated in 2.
