Mercenary companies, as a quick example, would generally be a mobile force of experienced soldiers who would undertake a contract for a fixed period of time or to achieve a specific objective.

These companies would be ideal for someone wanting to leave the town/region they were currently living in and get paid to fight someone else's war, or guard convoys, merchant trains, etc.

These companies were also ideal for criminals and otherwise persona non grata.

On the other hand, lets say you are a Lord or Governor(etc) of a town/city/port/region.

The safety of the people, animals, the land itself (for farming purposes, vital for prosperity) along with any trade routes that ran through such a town/city/port/region, along with any other "valuables" (such as guard posts, castles, etc) were all your responsibility.

If you failed this responsibility, you were removed from office or exiled, or simply just killed. This would be done by your liege, such as a king, or by the government that you governed for. It would also be reasonable to beware of the local population, as they would eventually grow tired of you and that typically led to massive instability, which weakened your standing and made it easier to invade your slice of the world.

Which brings me to my answer to your question.

A lord, lets say, has an interest in projecting a safe environment and protecting their assets, such as people, livestock, etc.

This was done by employing guards, or men-at-arms.

The lord would supply them with training, equipment, food and water, lodgings and lastly an allowance.

This would be EXTREMELY useful for the average male person who lived in the town/region who did not have a trade or was typically more adept at fighting then other people.

It would provide them with social standing, purpose, money, food and accommodation.

It would also help in lowering the amount of bandits in your territory, because you have all the best fighting men, you pay their wages and keep them in line and they in turn protect your assets!

As an average man-at-arms, you could expect a hard life. However if you were generally of average intelligence, loyal to your employer and had the respect of the other men-at-arms, you could expect a range of perks.

Moreover, if you managed to ingratiate yourself with the sergeant-at-arms or anyone of higher standing then yourself, such as even the captain of the guards, then eventually you could expect, after several years, (think ten to twenty years, unless you distinguish yourself in some event) promotion. This could be for a range of different positions, depending on your abilities. Positions such as quartermaster (the paying of wages, including the care-taking of equipment, ensuring the men were fed, generally the care-taking of the guards, was another prized role for the more intellectual. Sergeant-at-arms roles would be designed for those with leadership and disciplinary abilities.)

Promotion could be achieved many ways, firstly, service. Secondly would be again, distinguishing yourself as capable. Thirdly is the simply fact that people die, they grow old, they lose their edge. This opened opportunities for advancement, if you had made strides towards such a goal.

Overall the life in the trade of a professional guardmans was not ideal for family life, as you would be quartered with the other guardsman and sleep in the barracks. However with time, if you advanced to sergeant-at-arms, you could possibly expect your own room, more pay, better equipment and most importantly; Favour from your superiors.

Hole this helps, feel free to add or correct me.