Variable gain preamp ?

Can someone please let me know what the benefit of this is . I am looking at a 2 dodd audio pre's . one has the variable gain option . it is 1,000 more . is this worth it . why would i use it and how often would i be turning that dial ? my speakers are proac response 2.5 's . I am going to purchase one of these pre's then get a new amp . will the amp be part of the decision on what pre i would get ? Thanks,mike