ets2 and ats have these world of trucks (wot) external contracts that become available when you connect your game to the wot system.
what is the point of these external contracts?
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There are three points I'm aware of:
Every now and then SCS runs Community Events that require you to fulfill external contracts (in this case, the Operation Big Sur requires that you complete at least 15 deliveries from or to the construction site on Route 1 in American Truck simulator). Please note that Community events are optional.
"World of Trucks is envisioned as an optional layer created alongside the existing single player game." - SCS Software
Source: http://blog.scssoft.com/2015/12/world-of-trucks-contracts-questions.html (very early Q & A).
Additionally to the things pointed out by Gerald, a few other things are,
A few more points not covered by other answers (possibly because of changes since then):