Why did the Supreme Court allow hearsay evidence to be used to establish probable cause, when it would have been inadmissible at trial?

Mr. Justice Whittaker delivered the opinion of the Court
The evidence offered at the hearing on the motion to suppress was not substantially disputed. It established that one Marsh, a federal narcotic agent with 29 years’ experience, was stationed at Denver; that one Hereford had been engaged as a “special employee” of the Bureau of Narcotics at Denver for about six months, and from time to time gave information to Marsh regarding violations of the narcotic laws, for which Hereford was paid small sums of money, and that Marsh had always found the information given by Hereford to be accurate and reliable. On September 3, 1956, Hereford told Marsh that James Draper (petitioner) recently had taken up abode at a stated address in Denver and “was peddling narcotics to several addicts” in that city. Four days later, on September 7, Hereford told Marsh “that Draper had gone to Chicago the day before [September 6] by train [and] that he was going to bring back three ounces of heroin [and] that he would return to Denver either on the morning of the 8th of September or the morning of the 9th of September also by train.” Hereford also gave Marsh a detailed physical description of Draper and of the clothing he was wearing, and said that he would be carrying “a tan zipper bag” and habitually “walked real fast.”