Why did the Roman government begin hiring foreign mercenaries in the 3rd century CE?

One of the
reasons that the Roman government began to hire foreign mercenaries around the third century was
because they needed to fortify their borders. The government made contracts called
foedera with those they considered frontier barbarians who were living
beyond their borders and were not under their imperial control. These mercenary groups
essentially functioned as a sort of buffer between the Romans and other groups living along and
beyond the frontier. This was often a temporary measure, meaning that they were recruited when
the need arose, usually for immediate purposes. These particular soldiers always remained
somewhat on the outside: they served under their own leaders and beyond the borders of the
empire. However, they occasionally received Roman honors and ranks.

Another
reason is that certain mercenaries had special military abilities that made excellent additions
to the Roman army. For example, Hun, Alan, Bulgar, and Avar mercenaries made excellent mounted
archers,...

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