He was convicted because fibers found on the body of one of his victims matched fibers from the carpet in his house. But they are great evidence and can be used to determine time of death. In some cases, they can be used to determine if a body has been moved from one geographic location to another. Certain bugs incubate and hatch at certain known rates. If bugs are found on a corpse, the age of the bugs can be extrapolated backward to estimate time of death.
Blood spatters help a great deal in recontructing a crime scene.
They can be used to corroborate or disprove and alibi. They can be used to convict the guilty. The patterns of the spatters and the shapes of the individual blood droplets themselves can tell how the crime was committed. A drop falling from a low height of a few inches will leave a small cohesive circle. At greater heights, the circle will be larger and may even have a 'crown' effect.