1. High Availability: Redudant Routing Engines(REs).
In a VIrtual Chassis configuration, one of the member switches is elected as the master switch and a second member switch is elected as the backup switch. This design approach provides control plane redundancy.
2. Single MGMT access for entire VC, So its reduced operaional cost.
3. Single Confiuration for all switches.
4. You avoid the STP( RSTP,VSTP,MSTP) all ralated spaning tree for entire one VC.
5. Backlance throughtput will be more.
And for both switches you will have single configuraion file and one MGMT Ip address to login and its deffault for both switches in VC.
https://www.juniper.net/us/en/local/pdf/whitepapers/2000427-en.pdf