Learn English – What does “head first” mean

The meaning doesn't change, but there is a subtle difference in the nuance, or "voice of the phrase".

What use is lambda in PHP?

Sounds to me like the speaker is skeptical of the benefits of using lambda in PHP. They probably know how to create solutions using different methods and don't need to use it, but they are curious about why others would use it instead of something else, or perhaps in a blunt manner, the speaker wants to know why anyone would use lambda instead of using whatever the speaker uses. "What use is..." sounds somewhat negative/pessimistic.

What is lambda used for in PHP?

Sounds more neutral and suggests the speaker is looking for a general run-down of everything that lambda can be used for in PHP. Not necessarily pros or cons, just what parts of PHP it is generally used for. It doesn't suggest that they use alternatives, or that they know of alternatives to using lambda.

In the given context, Alex Jones is trying to belittle John Oliver by saying him "boy". It is derogatory, if you call a fully grown-up man a "boy".

From Merriam-Webster

an immature male

separate the men from the boys

Also, it is an offensive¹ word for a male servant.

^{1: Check Entry 2 in Merriam Webster}