Why did some computer cases have an additional door covering the

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Zalman Z1 Plus ATX Mid Tower PC Computer Case - Tempered Glass Side Panel - 3 x 120mm Fans Pre-Installed

4U Interchangeable Rackmount Chassis with Front Accessible Expansion Slots Computer Chassis · Power Supplies · ServerNAS · Storage Devices · Coolers · Fans Cable for motherboard with 2 USB-A 1 USB-A USB C3 1 1 Audio microphone jack fit CDDVD Room Slot at

aruba 3810m 48g 1 slot switch Zalman Z1 Plus ATX Mid Tower PC Computer Case - Tempered Glass Side Panel - 3 x 120mm Fans Pre-Installed Hi, i'm looking for a modern case that can house a optical drive, since it has become much rarer with time; Preferably = 100€