Gods people want everyone to have access to good judges. To ensure this happens, we should provide financial support to the best judges, so that they can handle more cases. Paul encourages this giving in Roman 13:6, but his meaning is lost due to the way the passage is translated. Most modern translations insert the word government, which is not in the Greek text. Here is a more literal translation:
For this reason, you also pay tribute: for they (good judges) are God's servants, attending continually to this very thing (Rom 13:6).
This passage is not a justification of general taxation by governing authorities. Payment should only be made to Gods servants. In this context, Gods servants are the judges who are busy hearing many cases, and especially those dealing with many appeals. They are the excellent judges who working continually on the thing that Paul is talking about in this passage: applying Gods law to disputes between people.
We want them to be available for this task, so that sin is restrained and society remains peaceful and harmonious. For this reason we should make voluntary payments to these judges, so that they can work full time at their work. A tribute is a payment made by one person to another, or one nation to another, for protection. Paul is saying that those who do not need a judge in their current situation, can submit to and excellent judge by contributing to a retainer for that judge.