Sometimes it may be useful to create or adjust a keyframe while an effect is disabled. I'll give an example:

I often work with video that has small logos that occasionally appear near the bottom or sides of the screen. I use motion keyframes on anchor point and scale to crop these logos, and I use ease in/out transitions from the default values to the final crop level to create an unnoticeable zoom/cropping motion.

However, once I've created the crop effect I want, I need to then disable the motion effects to determine where the crop needs to start and stop on the clip. However, once the effect is disabled, I can no longer create a new keyframe because it's grayed out. So I disable the effect, scrub to the position I need, re-enable the effect, create keyframe, and then disable the effect and repeat for the end of the crop. It's very irritating to work this way, and Premiere used to let us modify keyframes while the effect was disabled.

I'm sure there could be other scenarios where modifying keyframes while the effect is turned off can be useful as well. After Effects still lets you make adjustments when the effects are turned off. I don't know why the behavior was changed in Premiere.