If you only have 1 WAN connection and the slidder isn't set to the proper value , the MX won't be able to know the link speed and may try to push more than the link can handle which may result in latency / packetloss. If you have more than 1 WAN connection it is also used to load balance traffic according the speed of both WAN. https://documentation.meraki.com/MX/Firewall_and_Traffic_Shaping/SD-WAN_and_Traffic_Shaping
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