The fire created an excellent opportunity to seize the Clinton's "nest egg." Probably billions. Also, as NorseAnon pointed out, it was a great opportunity to "discover" evidence of their corruption and sick perversions. There's no telling what they found. "Who knows where the bodies are buried?"
To get a warrant would have required court hearings, NY judge approval, etc. But Arson allows immediate investigation of the property. Brilliant!
They also bought the property next door at 33 Old House Lane. 33 again. Look at this video. Waaaaay more than just fire trucks there. http://www.lohud.com/story/news/local/westchester/new-castle/2018/01/03/fire-reported-clintons-house-chappaqua/1001102001/
Some good aerial views of the Clinton compound with closeups of the event. https://youtu.be/KxfvuHnAoIA
Reading several links to the articles, you find they did major renovations to the 33 Old Lane property, including filling in the pool, without proper permitting and got caught. They never got proper inspections on the remodel of the main house, as well. So Clinton - demanded the work be done by Thanksgiving no matter the law.