The advantage was that, should there be leaks in wars, there would be ready people to deal with them as soon as they entered. They could also reassign guards elsewhere where they'd be more needed.
In the ideal settings, some areas could be mostly defended by mercenaries, just with some guard teams organizing and bellowing out orders.
Not to mention, having huge amounts of mercenaries as mainstays also increased the territory's 'overall strength', which worked a lot as credit but in wars. The sheer amount of mercenaries would increase their cap for allied forces in wars, just by existing.
This was especially the case if these mercenaries shifted their residencies and became true Alterrans. Powerful mercenary teams were not exclusively and strictly tied to their territory of origin or the territory wherein their main base stayed, so this was not an uncommon occurrence; it was just that this move was usually done from a lower territory upwards.
