Well, to act as bastions of Greek control then. The Roman ones had that aspect; as much as they were intended to be a civilising influence, they were also full of retired soldiers from one of the most hard-bitten armies out there, who could theoretically be called up in the event of a revolt or foreign incursion - as indeed the veterans of Colchester were when Boudicca came calling, for all the good that it did them.
I don't think that's particularly good of a characterization either. There was no obvious logic to the dispersion of
kleroi. You'd assume, for security purposes, they'd be concentrated around the frontier, or something, but they weren't. (The Ptolemaioi, for instance, did not place many
klerouchoi in Syria-Koile or Ioudaia; most of them were in the Herakleopolite and Arsinoite
nomes.) There aren't really any textual instances of
klerouchoi being called up as the equivalent of reservists, either; they were the mainstay of the army, not a second-line delaying force.
I'd say they were more like Byzantine thematic soldiers. Both were soldiers settled on plots of land that could be taken away by the state if they failed to turn in their military service, and simultaneously provided with tax breaks to allow the flexibility of being able to both farm and go on campaign. Like
klerouchoi phalangitai, the thematic soldiers served as the backbone of the Byzantine military, although the introduction of
tagmata and the movement of Byzantine borders eastward eventually erased their usefulness.
The comparison isn't great, obviously.
Klerouchoi were provided with slaves such that they didn't even necessarily need to actually do the farming, giving them as much flexibility as Alexander's foot-companions, and their relative wealth also probably gave them the time and cash to be well trained and consider fighting more of a profession than a hobby or obligation, and were consequently of better quality.
