I believe that pay tournaments on online sites, such as Bridge Base Online, are a great learning tool for an advancing player, either with another peer player, or with a mentor/coach. The tournament itself provides structure and rules, and has a Tournament Director to resolve problems (btw only play in pay tournaments where the TD is not playing, so they can devote their attention to the task at hand, instead of to their hand!). The tournament provides results every two or three boards, and thus the players get immediate feedback for a really bad or good move at the table.

While waiting for the next round, players can use the time to review boards, seeing the four hand layout, and discussing. In live bridge, laying out all four hands to talk about it doesn't work that well, especially since some eavesdroppers at the next table are about to play the hand. Certainly in live bridge one can review the boards after the session, but sometimes one doesn't have hand records, and one's thinking is no longer fresh in mind. Also one often has to transcript the results to see where the big gains and loses were, while online play provides this feedback as it happens.

In addition, if playing with a mentor, online play allows the advancing player, if dummy, to follow partner's play. In particular, they can decide on what their plan of attack would have been, if they were declarer, and then see what line partner takes.

What about online play outside of tournaments? - it can still be quite valuable but does not provide much time between boards for review - usually the opponents expect one to move right to the next board, without any unnecessary chatter.

So in short, online pay tournaments are a great development tool for advancing players.