Noisy environments, requires robust equipment. Down time costs money, and schedules. It is not always practical to have the highest noise immunity, but equipment must match the conditions it will be used in.
Linking to the real world comes in many formes, from boards in Rack systems to stand alone units. VME bus to relay control of external devices. RS232, RS422, or RS485? Selection of the most appropriate can be a key to the future success of a product.
Design advantage... cheap, simple. 16C84 has some non volatile memory on board which is ideal for remembering setup conditions.
Complex logic devices help improve development timescales, and reduce pcb space (compared to standard logic devices). They also help reduce cost as PCB revisions are reduced. Excellent in low and medium volume development. In System Programming allows modification of SMT device logic with reduced risk of damaging the device. These devices are well suited to in circuit testing to assist with reliability. Used for signal conditioning after analogue filtering and mopping up general logic on board.
Gain space from surface mount devices, and allow use of dual in line devices when nesessary. Most board manufacturers have their own preferances. They will depend on volume, cost, reliability, test methods.