I will answer your questions in reverse order. This is definitely an excellent opportunity for you. Being invited to be a reviewer or editor is an honor and can be beneficial in many ways.
1. It is a great addition to your CV as this means you are considered an expert in your field.
2. Evaluating scientific papers will increase your knowledge. Reading other people's work critically can even give you great ideas for your own work.
3. You will be recognized by the editorial board members who are usually senior professors and scientists. The scientific fraternity will recognize you and value your contribution.
4. Lastly, you get immense personal satisfaction as you are contributing toward furthering science.
You should definitely accept the offer unless you have strong reasons for doing otherwise. However, before you reply to the invitation, do a bit of research about the journal. It is fine if the journal is not indexed in a database, but it should have a good reputation. Ensure that it is not a predatory journal since you would not want to be associated with disreputable journals. You can use Beall’s list of predatory journals for reference. If it is clear that the journal is a reputed one, you can surely go ahead and accept the invitation.