Verse 24 at first glance appears superfluous, but it actually shows an awareness of what Mark (Mk 1:14) had written, and John is making clear that these events took place before the imprisonment of John the Baptist.
We now come to the handing over of the baton from John to Jesus. John’s role is almost complete, and Jesus’ role is about to become much more prominent. When God’s plans move to a new phase there is always some dispute and discontent, for people become comfortable with the situation they are in, and fear change. So it was here. The “argument” is actually a debate, and it may be that John’s disciples started the debate. Jesus’ disciples were not over careful about things like ceremonial washings, and perhaps John’s disciples, feeling threatened, wanted to use this to prove that they were better than Jesus’ disciples. It is a common human tactic to pick some irrelevancy and seek to blow it up into a major issue.
John’s disciples then went to John himself, seeking to enlist his support. They reported that Jesus was baptizing and attracting larger crowds than John. “Someone has opened a new church and everyone is going there!” Notice that they said “the one you testified about”. Evidently they had heard the words, but had not understood a thing. This is another common human trait. You can preach or teach a message and then wonder if anyone heard a word you said, or they seem to have been listening to a completely different sermon. People will filter whatever is said.