Physical evidence, in any case, is essential. Physical evidence like forceful entry into the structure, windows left open, or interior doors tied open in an arson case could show that another crime was committed besides arson. An example would be firefighters dispatched to a fire but noticed that windows were open and no one was found inside. That could mean that the house was intentionally set on fire, and the arsonists escaped. On the opposite side of things, if no evidence was found on the scene and it was just a regular house fire, arson investigators would investigate where the fire started and what could have started the fire. Physical evidence from a firearm would be beneficial in an investigation. Fingerprints and ballistics can be obtained from a firearm. If the firearm or cartridge cases are not found, it would be harder to determine what kind of firearm was used, and fingerprints would be impossible to obtain. In drug cases, I feel that without physical drugs or loads of money from drugs, the case would be complicated to solve. In some cases, witnesses would be very helpful, but even then, it is not 100% guaranteed