The main reason for adding this parameter in release 9 was to avoid/neglect the signalling of the serving cell.
I mean to say, before it was a headache of serving cell to configure the time periods in which a UE can decode the CGI or system information of the target cell.
Autonomous gaps: These are GAP periods where UE will stop the Tx/Rx of the serving cell for a moment to perform measurements or to read MIB/SIB of the target cell.
if UE supports this autonomous gaps and network is also aware of that so it has to just set it to true or false.. si-RequestForHO is set to FALSE/true otherwise UE will use IDLE periods.
Please note that the maximum length of autonomous gap is:
0.15 seconds for an E-UTRA CSG cell
2 seconds for an UTRAN FDD CSG cell
The cell for which the UE is being requested to obtain the CGI will be indicated by
cellForWhichToReportCGI in the measurement object of the concerned target RAT.
Now the last query in which scenario it would be useful:
The reports given before was not sufficient for the Scell to make the handover decision so it can ask from the UE for the additional information. No need to configure the DRX time duration again. Just make this si-RequestForHO true and UE will give you the report.