Why the FBI released the FISA warrant on Saturday night

Because it's full of really bad news.

First it shows that the FBI vouched for the truthfulness of the Steele dossier when they had not in fact verified it.

Second is shows that they did not tell the judge that the DNC funded Steele.

Third it ignored standard procedure.  When a cop has a Confidential Informant(CI) who has dug up information in order for the court to trust the data the prosecutor has to show that the CI is trustworthy.  It doesn't matter how trustworthy the cop is;  he can't vouch for the CI.  The CI has to be identified and reason for their trustworthiness has to be shown. But in the FISA warrant the FBI used the "trustworthiness" of Steele because the Russian sources he used were never identified.  Hence if this were a regular criminal case the fact that the Russian sources were unidentified and they weren't even speaking of events they had direct knowledge of, they were all talking about things other people supposedly told them, there is no way a wiretap warrant would have been issued.

Fourth the FISA warrant cites news articles as sources.  One article we now know was based on Steele talking to the reporter who wrote it and hence can't be an independent corroboration of the Steele dossier. The other was even worse. It was an article from the Washington Post which claimed that Trump has weakened the GOP platform position on the Ukraine.  Yet the reality is that Trump strengthened the position.  So the FBI used as "evidence" an incorrect story published by the highly partisan Washington Post to turn the full power of the US intelligence community against the Trump campaign.

Fifth the document mentions that Carter Page had been approached by Russians in the past but it does not appear to mention that Page cooperated with the government to help the government arrest those agents.  It hardly makes sense that someone who cooperated in catching Russian spies a few years ago would suddenly decide to work with the Russians so it would probably be the sort of thing that the FBI wouldn't want the judge to know about.  Of course the information can be somewhere in the redacted text but why?  It's public knowledge that Page worked with the government against the Russians so there should be no reason to redact that.

Basically even with much of the document redacted it's clear that the FBI was playing fast and loose with the facts in order to get a FISA warrant to spy on someone related to the Trump campaign.

If the FBI wasn't trying to get Trump they would have warned him about the people associated with his campaign the FBI was interested in so Trump could have either stopped having his people talk to them or help the FBI catch them, as Page had helped the FBI previously. By not contacting Trump the FBI made it pretty clear that he was their target.

We're looking at something much worse than Watergate. The difference is that when Watergate happened the Republicans refused to back Nixon whereas now the Democrats are backing the FBI/DOJ deep state.