There are reasons for this.
Firstly its about trying change the nature of local councils, even Labour ones, which implement Tory policies not through necessity but by choice. The only way to fight this is to get better candidates selected to stand but this requires internal work as selection committees are in the hands of individuals who are basically Tory in outlook.
Then its a recognition that if you want good policy or party unity the only way you can do that is to make sure that the majority are properly listened to. This means you need to control party structures from the bottom up, which is exactly what Blair did when he was in charge, except of course he did this to stifle democracy rather than enhance it. Failure to do this, will result in the PLP toppling Corbyn and then our country will have no genuine choice again in who runs the country. Failure to control wards, means failure to control GC, which then means failure to control conference, policy forum, NEC, local councils and therefore failure to control policy and with it potential political outcomes. It's a sad truth that no matter who is leader, without structural changes, the Labour party would deliver nothing if in power.
One final point, take the issue of Brighton, which was featured in the expose programme, where you would have heard Momentum people use terms like 'Traitor'. This is a CLP which overwhelmingly voted to back Corbyn with a turn out of thousands. The NEC then suspended the entire CLP. Like the entire constituency Labour Party in Brighton Hove! Rather than the MP for Brighton Hove calling this an outrage, he supported the NEC's decision. This is why they want to deselect him. He supported the undemocratic dismissal of his entire local membership. They are furious at him. You cant go to war with your own local members and expect them to back you. These kind of shenanigans are happening across the whole country. I've seen friends of mine suspended for doing absolutely nothing wrong. More people got suspended from Labour unfairly than voted for Owen Smith in totality. More than 100,000 people didn't even get sent their ballot for the recent leadership election. It was a rigged election, even if Corbyn won.
Genuinely, if we just ignored internal matters, the PLP would have already toppled Corbyn and we'd go into the next election with a Blairite, and you'd see a huge bleeding of members, and the Labour party would die, and the Tories would be in power for another decade or more.
Despite all of this though, our main focus is still external. Most of what we do is not bitter in fighting, believe it or not.