It attracted the attention of law enforcement. The possibility of the
existence of Russian kompromat, coupled with Trump’s weird rhetoric and bizarre behavior toward the Russians, raised the specter that the President of the United States was being
extorted by the Russian government. This possibility
required the FBI to open a counterintelligence investigation of the president.
“In the days after President Trump fired James B. Comey as F.B.I. director, law enforcement officials became so concerned by the president’s behavior that they began investigating whether he had been working on behalf of Russia against American interests, according to former law enforcement officials and others familiar with the investigation.” — The New York Times
F.B.I. Opened Inquiry Into Whether Trump Was Secretly Working on Behalf of Russia - The New York Times
So, it doesn’t matter if the British agent was washed up. Nor does it matter if Trump is perverted. The concern is whether or not the President of the United States is under the influence of a foreign, hostile government. If you think our law enforcement shouldn’t investigate that possibility, then...well, that seems pretty stupid.