Rear gelatin filters are mainly useful for simple filters that don’t need adjustment, such as solid ND or color correction gels.

Advantages of rear filters:

• Smaller filter size, so they’re lighter, easier to carry, and can be cheaper.
• Gel sheets are often sold in sizes that can be trimmed to fit a rear holder more easily than covering the front of a lens.
• The filter is protected inside the lens mount area, so it’s less likely to get scratched.
• Less risk of added front-end vignetting, and you may have more flexibility when stacking front filters.
• Essential on lenses where front screw-in filters aren’t practical, such as some fisheyes.

Disadvantages:

• You usually have to remove the lens to install or change the filter.
• Not suitable for filters that need to be rotated or adjusted, such as circular polarizers or variable ND filters.
• Gelatin filters are less durable than glass or resin screw-in filters.

So, if you need convenience, toughness, or a rotating filter, front filters are usually better. If you want a compact, protected way to use simple cut-to-fit filters—especially where front filters are awkward—the rear gelatin holder is useful.