Without warm-up period, the simulation model would start empty (no products at any of the machines), but real factories almost always start with work-in-process inventory. A situation where you would not need a warm-up time in a simulation might be when you are simulating a situation which starts empty each day: like a shop, or a parking lot. However, even in these circumstances you would probably want to separate off the results you get during the early part of the day from the results under peak conditions because knowing the 'average' performance of an operation which never works under 'average' conditions is probably not worthwhile. Most simulation packages contain facilities to set a 'warm-up' time during which results are either not collected, or which can be separated off from the main results collection period.
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