What will writing essays about how an author presents this-or-that add to her enjoyment?
But surely you could apply that to any subject? Why write up experiments in science, why do lots of maths questions in exams, why write history and geography essays...the list is endless.
As a former English teacher too, I'd say there is huge value in analysing texts- it's why adults join book/ reading clubs, Radio 4 has a Bookclub programme- because readers like to discuss novels and how there can be different interpretations of characters, plot, language, etc.
You cannot do this orally with 150 kids each year for GCSE so they have to write it down. Exams test understanding. Some kids might be turned off having to 'read to order' but equally others might just find something they enjoy and become lifelong readers.
GCSE Lit is the basis for A level English, so are you also saying that it should cease to exist- and BA/MA/PhD English too- because that's the logic of your argument it seems.