Why Hire Adventurers?

[Witty Username]

Regular military forces in feudal/medieval period is a bit of a backward pushed anachronism: the 'regular' were available usually for about 40 days per year. It's a limited resource, not the always on regulars that we see in 20th and 21st century nation states. This is related to my point above.

Adding to this, if this is a local lord like the prompt suggests, the regular forces might be as many as 20. Usually enough to loosely police ones holdings and keep your tax collectors safe. A proper warband would nessitate outside help, either from neighboring lords, mercenaries, or pesant conscripts. All assuming the local lord hasn't been called to respond to a war of some sort (send all your knights 6 to 8 months away for a few years, and get back maybe half of them).
In the mind of the lord, hiring adventurers could be a matter of buying time for their allies to muster, to their only viable option depending on their surrounding circumstances and obligations.