Why referee?
There are several reasons why researchers should be willing to provide referee reports.
You learn a lot. If the paper is in your area, then writing a referee report forces you to read it very carefully and engage closely with the research of other people in your field. There’s no better way of understand what is going on in your field.
You get better known by the research leaders in your area. It is essential to your research career to develop an international reputation for a high standard of scholarship. Once known, you may get asked to submit an invited paper to the journal, become an associate editor of the journal, write a commentary on another paper, etc. Opportunities will open up if you are known to be a good referee.
You get to see the latest research before everyone else. Often, an author won’t release a working paper or pre-print before the paper has gone through a round of refereeing (and some authors keep things to themselves until a paper is accepted). So you get a head-start on everyone else if you referee the paper. It might lead you to develop some of your own ideas and write a new paper that builds on the results.
If you submit papers to journals yourself, and benefit from the referee reports that you receive, then you should be willing to do the same for others. The whole system is built on researchers providing a mutually beneficial service, and if you want to participate in the system then you should be willing to contribute to it.
On the other hand, you do sometimes need to be selective. Politely decline if you think the paper is not close enough to your own interests to be worth spending time on, or if the journal is not one you are likely to ever publish in, or if you don’t feel capable of understanding the paper well enough, or if you have already written three reports in the past three weeks. If you do say no, it is very helpful if you can recommend someone who would be suitable.