It is part of the anti smog system on the 4.2L. It isn't needed unless you have to test for smog or emissions testing. But it isn't required for the motor to run properly.
'87 YJ, 4.2L built and bored, 8" of lift, 8.8 rear, AX15/NP 231J, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...