Why is sedmentary rock formation important to fossil formation?

Fossils are found in sedimentary rocks.  Usually a fossil
is a preserved specimen of some living organism.  Therefore the assumption is that the
organism was buried in some sedimentary environment.  The development of the sedimentary
formation then affects the fossil development.  If the sediment has the proper chemical
conditions the original life form is replaced, atom by atom, with a new element.  For
example the carbon atoms are replaced by silica and the resulting life form has all the
original shape and structure, but the chemical composition is altered.  I have seen tree
branches that extended through severall layers of coal.  The structure was maintained,
but the composition varied, one section being carbon and the two adjacent sections were
sandstone.  The branch structure remains clearly identifiable continuously throughout
the structure. If the development of the sediment is harsh, we find fossil fragments.
Not all sedimentary rock formations contain fossils, but those that do suggest
information about the history of the development, and occassionally indicate information
about the environmental conditions during the deposition of the
sediment.