Yes it is essential work. The Aircraft in storage require various checks.
The storage checks required depending on type are 3, 4, 7, 10, 14, 30, 60, 90 and 180 Day checks. Each higher level check requires the lower level checks to be completed as well in most situations.
There is more work now for the Line Stations than there was with all aircraft flying with not enough people on shift. With some aircraft requiring Return to Service checks as well which has an enormous amount of work.
This is the argument from the ALAEA. There has always been useful work for LAME’s.