In GT6, part of the function of the cover is replaced by the sensor of the block. Although this means that there are more functions, the sensor is a separate block rather than an accessory of a tileenity, which makes some automation not more compact (for example, the detection surface is directly attached to another block rather than across a block), Therefore, the regression of some cover may be useful.
Item detection cover: it is replaced by the Item-O-Meter Sensor in gt6. However, the Item-O-Meter Sensor block cannot detect items in a specified slot or a certain item in the container. Therefore, I think this kind of cover plate is useful.
Fluid detector: same as above, also for fluid
Player detector: it can detect players other than the placer and the placer. The cover cannot be removed by players other than the placer. In the server, you can prevent others from using your machine to “cheat” and skip some techtree.
Fluid Regulator: similar to electric pump, but can accurately control flow. It may not be as useful in gt6 as in gt5, but it may be useful in some automation (such as controlling steam diversion).