If You create fewer or equal Standby Redo Log (SRL) groups than Oracle Redo Log (ORL) groups, then you may run into trouble when the primary has a high rate of redo generation, especially if the primary is RAC db. You should have enough SRL groups so that the Network Server SYNC(NSSn) process involved in maximum protection mode and Network Server ASYNC(NSAn) process involved in maximum performance mode can write from all of the ORL groups from Primary database to SRLS at Standby database.
For better understanding purpose consider below scenario:
In standalone (Non-RAC) DB, if the primary DB has 2 ORL groups #1 and #2 and redo switches are high due to heavy DML activities , in that case, we want to make sure that standby DB can keep up with the primary. If LGWR on primary just finished #1 and switched to #2, and now it needs to switch back to #1 again because #2 just become full, the standby must catch up, otherwise the primary LGWR cannot reuse #1 because standby is still archiving the standby's #1 SRL. Now, if you have the extra SRL group #3 on standby, then standby in this case can start to use #3 while its #1 SRL is being archived. That way, the primary can reuse the primary's #1 without delay.
Nice article on Why and How SRL by Brian Peasland: