The branches of government in the US don't work the same way they do here in Canada.  Trump's popularity numbers matter because if GOP Congressmen or Senators running for re-election in 2018 or 2020 think Trump will drag them down, they will distance themselves from him and vote against his initiatives to increase their chances of re-election.

If Trump is being investigated by the FBI... and if other branches of the government can disprove Trump's wild accusations, then he loses credibility with swing and undecided voters.  When that happens, his numbers drop.  When his numbers drop, he loses votes and his initiatives lose support.

That's why things like this matter.... the fact that you don't understand it is irrelevant.