You’ve acquired an antique or vintage mechanical watch that you would like to use. You wind it up and it starts and keeps good time. Should you set it and start using it? If the watch has monetary or intrinsic value then here is a brief explanation why it probably should be be cleaned and lubricated prior to use. (Related: What is the cost to service a watch)
First, let’s discuss the mechanics of a watch (This applies specifically to a mechanical watch that needs to be wound up, not a battery operated quartz watch). These watches run off of a wound up steel spring. When wound and running this spring exerts a relatively large amount of sideways pressure to the pivots on a series of gears (called the power train). As it runs these pivots are subject to friction and wear if not kept lubricated. Even if the works (known as the movement) looks clean and runs fine, the oil and grease evaporates and thickens over the course of several years even if the watch has not been wound. If the watch is run in this state, this increased friction will wear heavily on the pivots and jewels and eventually require costly repair. The parts that control the setting of the hands get high pressure wear each time the stem is pulled out (or lever) and will start grinding against each other as the grease ages.
If you purchased the watch from a seller that you trust, and they guarantee that it has been serviced recently then it probably is OK. If your not sure, then at least have it looked at by a experienced watchmaker. Once it has been service it should be good to go for 5 years or so.