It defines a clear cut boundary you can use to determine range (particularly in house games with in-house rules for measuring distance) at a purely objective level.
Also, it removes the rare cases of douchebaggary when people 'hide' their characters underneath a defiler/triarch stalker or any similar models that have negative volumes that do not normally have a base.
In short, having bases make models that you'd normally measure to hull conform to the core rules that much more objectively. No more 'YOUR ANTENNA IS .0000000000000000000000000000000000001" WITHIN RANGE OF MY GUN!'.