Reviewing papers can take a lot of time and effort, but also has the following benefits:
It is a good way to build reputation and relationship with the research community by writing an informative opinion on time and participating in the discussions to reach the final recommendation.
If you submit to a venue, it is only fair to review for that venue, especially if you have complained that your submissions have not received good/fair reviews due to lack of expertise or efforts from the reviewers.
It is a good way to learn about the latest research, as you might not feel pressured to keep up to date with the latest research in your day job otherwise.
Writing a good review (see above) requires deep enough understanding of the paper beyond the usually cursory glance at the published papers.
It is a good way to learn how to get your own submissions accepted in the future by looking at how decisions are made and what the reviewers are looking for.
Accept only reviews that you think you have the time and expertise to do a decent job, and do not feel bad about declining reviews, preferably within a few days of receiving the request with suggestions for alternative reviewers so that the organizers can find replacements in time.
Other things being equal, prioritize review requests from higher-quality venues for which your reviews would tend to make a relatively bigger impact.