They're asking him questions they already know the answers to in order to give him opportunity to perjure himself, which he's trying to avoid.
That's not entirely accurate.
They're trying to get him to reveal what he and Trump talked about while alone on several occasions so they can build a case for obstruction.
Before a committee like that, there are only three possible answers: you answer the question entirely, you don't know the answer to the question, you take the 5th.
Sessions introduced a 4th today for the first time in history: The "I'm invoking executive privalidge for the president because the president may be incriminated by whatever I say" answer.
What that's actually done is two fold:
- It's proven beyond any reasonable doubt that Trump has a lot to hide.
- It's proven beyond any reasonable doubt that Sessions is well aware of at least some of what he's (Trump's) hiding.