What are the benefits of poster awards? Is being awarded at a conference really useful for one's career (e.g. when applying for a PhD or a job) or is it just scene? In other words: Can I say that having such award on my resumé is better than just nothing?
In my field the thinking is
It draws attention to posters from conference participators.
It provides encouragements to individuals (students) to make an extra effort
It encourages discussion about "good poster design"
The selection criteria is always focusing on good communication of good science, never one or the other. Since such awards are given only to students and there ids usually also a similar student presentation award both selected by a set of valued senior scientists, the award should be and is seen as a merit.
Peter Jansson's answer provides a good description of the general benefits of poster awards mostly from the perspective of the conference organizer. My answer focuses on your latter questions.
A poster awards is definitely a positive signal of your quality as a researcher. If it was from a major conference (and perhaps even if it is not), it might do quite a bit to distinguish you from other prospective students in a PhD program applicant pool. Although its value while applying for non-academic jobs is more dubious, it can only there as well.
It is clearly better than nothing and you should put it on your resumé or CV.