To the onlooker, and particularly to those whose memories go back to pre-grouping days, the modern cavalcade of "V2s" and "B1s" is apt to become a little monotonous; but to any running man the general utility characteristics of these two classes are a perfect godsend at times of exceptional pressure, when it is often a case of "first in—first out" with locomotive allocations at sheds.
Source: wiktionary