There is an explanation at the end of section 3.3.
Aside from dot-segments in
hierarchical paths, a path segment is
considered opaque by the generic
syntax. URI producing applications
often use the reserved characters
allowed in a segment to delimit
scheme-specific or
dereference-handler-specific
subcomponents. For example, the
semicolon (";") and equals ("=")
reserved characters are often used
to delimit parameters and parameter
values applicable to that segment.
The comma (",") reserved character is
often used for similar purposes.
For example, one URI producer might
use a segment such as "name;v=1.1"
to indicate a reference to version 1.1
of "name", whereas another might
use a segment such as "name,1.1" to
indicate the same. Parameter types
may be defined by scheme-specific
semantics, but in most cases the
syntax of a parameter is specific to
the implementation of the URI's
dereferencing algorithm.
In other words, it is reserved so that people who want a delimited list of something in the URL can safely use ;
as a delimiter even if the parts contain ;
, as long as the contents are percent-encoded. In other words, you can do this:
foo;bar;baz%3bqux
and interpret it as three parts: foo
, bar
, baz;qux
. If semi-colon were not a reserved character, the ;
and %3b
would be equivalent so the URI would be incorrectly interpreted as four parts: foo
, bar
, baz
, qux
.