Perhaps mostly for balance?
Though the hammer head is small (2.26 x 2.79 cm. in maximum cross section), I imagine that it does move the center of gravity further back towards the haft (see photo below) and that the presence of the haft might well pull the center of gravity very nearly in line with the haft. Also, even if the purpose of the axe was primarily as a weapon, in the field a small hammer would occasionally prove useful.
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