From the CREATE TABLE manual page (emphasis added):
The fillfactor for a table is a percentage between 10 and 100. 100 (complete packing) is the default. When a smaller fillfactor is specified, INSERT operations pack table pages only to the indicated percentage; the remaining space on each page is reserved for updating rows on that page. This gives UPDATE a chance to place the updated copy of a row on the same page as the original, which is more efficient than placing it on a different page. For a table whose entries are never updated, complete packing is the best choice, but in heavily updated tables smaller fillfactors are appropriate.
From the CREATE INDEX manual page (emphasis added):
The fillfactor for an index is a percentage that determines how full
the index method will try to pack index pages. For B-trees, leaf pages
are filled to this percentage during initial index build, and also
when extending the index at the right (largest key values). If pages
subsequently become completely full, they will be split, leading to
gradual degradation in the index's efficiency. B-trees use a default
fillfactor of 90, but any value from 10 to 100 can be selected. If the
table is static then fillfactor 100 is best to minimize the index's
physical size, but for heavily updated tables a smaller fillfactor is
better to minimize the need for page splits. The other index methods
use fillfactor in different but roughly analogous ways; the default
fillfactor varies between methods.