It is usually a predictor of General Election results (whether accurate or not), and if Labour manage to end the night down would be a terrible sign for their prospects.
No, it isn't a predictor of general election results, it never has been. That's like saying that the European Elections are a predictor for Westminster, cos they ain't either, and never have been.
What these local elections are are the biggest electoral test of opinion since the last general election. Almost 250 councils, about 9,000 seats, and the Tories defending aroud 60% of those seats.
There won't be too much to be read into these results with regards to Westminster, more whether a strategy was a success or failure.
The word "used" should have been in that post (I have added it, it didn't make sense without it). To pretend that national results aren't read from these results (especially when in previous years national projections of what could have happened were it a General Election were made) is silly.
Some in the media read national results from these elections, but conclusions drawn from that tend not to be so accurate. The political parties on the other hand, don't tend to read national results directly, especially when, as this year demonstrates, there are almost 250 different sets of elections going on, with local issues, national issues, and even brexit all playing their part. About all they can tell is how well a national electoral strategy has gone down with their voters, and at this point, it looks like Labour and the Tories need to work on their strategies.
The fact you mention it's an electoral test shows there is a comparison to be made. If it were irrelevant, why mention it?
Not true at all. Every by-election, local election etc, is in fact an electoral test of some kind. In this case, it is the biggest test of national electoral strategy. But it's no more than that. Making a comparison with Westminster is at best statistically dubious, and frankly, a fool's game.