It was to get a work around of single-piloted wx mins.....the S-3 was considered multi-piloted for these purposes only.
The Prowler is considered multi-piloted even though there is only one set of controls.... hence, the NFOs did IGS and an annual instrument check. For the F-18D guys, the instrument check was optional... up to the CO if he wanted them to do it or not. Every NFO does IGS every year regardless of platform... even the E-2 bubbas.