Let's reverse the question and ask Why would PM give a bonus for a work? I am not sure what is your suggested reason for doing it, so let's consider some commonly used reasons:

• To motivate employees. Well, lack of money can be a very good demotivator. Adding more money doesn't bring motivation, it rather removes demotivation (final effect is similiar), but it works only to a certain point (where employee feels comfortable with base salary). Above that money doesn't motivate or does it very very poorly. Of course many people are subjected to wishful thinking that more money => more motivation, inc. productivity, better quality etc. It's wishful because we would like to receive more money for better motivation, more work or better quality. The reality is different and bonuses/rewards go to the better politics, better sellers, better negotiators. If one wants to mess with motivation, she should know another point of view.
• To bond employees with the company. The concept is: you work on the project - you receive a bonus, so you will more likely to work with us next time. It looks nice, but it's a monster with no legs. In the long term employees will much likely to choose better/nicer work environment with better motivation and cooperation. They can be easily bought by higher rewards of the competing company. And last, but not least it does not consider status of the project and indolence of the developers.
• To achieve the goals. I am not a fan of "selling" the goals. A healthy team shares the goals and tries hard to achieve them. PM shouldn't expect them to work over hours because it affects the long term productivity. But ok, there are moments when I can understand it. PM didn't notice some patterns early enough and she needs to deal with it. She needs to pay for every extra time or set a reward for achieving a goal. Maybe she promised something at the moment of better mood. It's maybe not the best choice but it works from time to time.
• Because she can. In this case, a bonus is not meant to influence the motivation, it is not a salary for work, and it is not a tool that helps achieving the goals. Not something that is "payed instead" or "payed for". It is a bonus - it is payed despite the other factors and it express the company's financial situation or particular happiness with project outcome. Like a gift.

Let's reverse the question and ask Why would PM give a bonus for a work? I am not sure what is your suggested reason for doing it, so let's consider some commonly used reasons:

• To motivate employees. Well, lack of money can be a very good demotivator. Adding more money doesn't bring motivation, it rather removes demotivation (final effect is similiar), but it works only to a certain point (where employee feels comfortable with base salary). Above that money doesn't motivate or does it very very poorly. Of course many people are subjected to wishful thinking that more money => more motivation, inc. productivity, better quality etc. It's wishful because we would like to receive more money for better motivation, more work or better quality. The reality is different and bonuses/rewards go to the better politics, better sellers, better negotiators. If one wants to mess with motivation, she should know another point of view.
• To bond employees with the company. The concept is: you work on the project - you receive a bonus, so you will more likely to work with us next time. It looks nice, but it's a monster with no legs. In the long term employees will much likely to choose better/nicer work environment with better motivation and cooperation. They can be easily bought by higher rewards of the competing company. And last, but not least it does not consider status of the project and indolence of the developers.
• To achieve the goals. I am not a fan of "selling" the goals. A healthy team shares the goals and tries hard to achieve them. PM shouldn't expect them to work over hours because it affects the long term productivity. But ok, there are moments when I can understand it. PM didn't notice some patterns early enough and she needs to deal with it. She needs to pay for every extra time or set a reward for achieving a goal. Maybe she promised something at the moment of better mood. It's maybe not the best choice but it works from time to time.
• Because she can. In this case, a bonus is not meant to influence the motivation, it is not a salary for work, and it is not a tool that helps achieving the goals. Not something that is "payed instead" or "payed for". It is a bonus - it is payed despite the other factors and it express the company's financial situation or particular happiness with project outcome. Like a gift.

Edit: To answer @Jonas question in comment:

I didn't even touch the motivation-subject because it is vast.

The motivation system can be connected with financial rewards (generally speaking - something company must pay for) but it should not play a major role as it doesn't have such influence on a employee's motivation. Design of such financial rewards in most cases will strongly depend on developer role in the organization.

For example: if developer is heavily involved with relations with customers (gathers requirements, provides support, install upgrades and makes customizations) and the overall image of the company depend mostly on developer's actions then a share of profits seems as a reasonable reward. But if developer is involved only in back-office actions and she works mostly in a team (like in Scrum) then the most suitable would probably be a reward based on the team performance or on the extraordinary teamwork events.

If one 'feels' a financial part should play more important role in a motivation system, then one probably needs to think about how comfortable she feels about her base salary.