Labour can try to bring down the government by a vote of confidence in the government. Which would (if successful) bring about a General Election.
As I pointed out, no it doesn't. Losing a vote of no confidence would kick in a period of 14 days during which the Conservatives could propose some other person who commands the majority of the house. With the majority they have, that should be relatively easy. The only person guaranteed to lose their position would be Johnson.

As to why? Well, there are several reasons but one that might appeal to the Conservatives might to show that they have found Johnson's behaviour unacceptable and rather than let him leave 'with his head held high' they think it would be better for the next leader if he was dismissed taking all the blame for parties and other misjudgements. It would help to sell the story that all the dubious behaviour was due to Johnson, and it is genuinely a rejection of that style of government.

I think that would be a hard sell, but that politics for you.