Hello Max
The mechanism and electronic sensors of an INS require warmup time for stability and accuracy - most units have temperature controlled ovens to mitigate thermal errors. STANDBY mode allows the platform to spin up and warm up without initiating the ALIGN process - a time saver when the aircraft is powered but may not be ready for full INS alignment.
IRS (RLG) units are typically ready to go directly to ALIGN within seconds of power up, with no thermal compensation or stabilization required.