It means 'hey, look at me'
con·spi·cu·i·ty (knsp-ky-t)n.
The quality of being conspicuous; obviousness.
Broadly speaking, used by aircraft not receiving a service from any ATC unit, or being provided by a service from an ATC unit without SSR. Used when no specific code has been issued by ATC.
Military aircraft in the low-flying system use 7001 so you know that they are not working an ATC unit and they might climb rapidly if the weather becomes unsuitable at low-level.