On this page:

The Account Dashboard gives a quick overview of how your account is performing. It lets you see overall account and campaign-level performance and performance segmented by network and device.

Compare how much traffic the account gets on different networks and devices. 

For example, see how search and display are performing on mobile in terms of impressions, clicks, conversions.

Identify traffic patterns in the graph, and spot any big drops or increases that happened.

Find specific campaigns and ad groups that need to be optimized for particular segments. Adjust the settings on the left, to get a list of campaigns/ad groups/keywords that accrue high cost on a particular network or device.