Several reasons actually.
The F-16 is a truly mult-role aircraft. Its very capable in both
its A2A and A2G role. Just that the pilot may not be as
capable.
Especially when taking on the Wild Weasel role, or SEAD role, a
single pilot will be thoroughly stressed when he has to handle :
Flying of the aircraft, firing of the HARM or Shrike, checking the
EWS for additional threat, and evading the attacking missile if
any. All that while trying figure out how many more secs til
missile impact. IE, calculating missile closure rate relative to
his aircraft's orientation.
So with 2 guys in the cockpit. The pilot worries about flying and
evading the missile, while the WSO can worry about other
threats.
Of course, I think the most fun the WSO has is guiding of PGM via
targetting pod, and recording some really cool BDA videos. Again,
the Pilot worries about flying, under the guidance of the WSO to
the release point, and the WSO keeps and eye on the bomb from
there. Fun huh?
2 heads are better than one.
Think about it, some of the most established aircraft in the
business of dropping PGMs comes with crews of 2. The Strike Eagle,
F-4Gs, even the F-14D Bomcats.
Btw I heard they were bringing back the SuperTomcat project. Anyone
can verify?