Dear Bala,
The aim of awarding punishment is correcting the behaviour. Your query is on issuing directly a warning letter directly. Yes, it can be issued but only for the minor offences.
However, before issuing a warning letter be clear that you have all the available facts or even material evidence of the misconduct at hand. There should be no room for assumption.
The benefit of the issue of show cause notice is that the company gives the accused employee complete chance to defend his/her position. By chance, if some issue is overlooked then there is a possibility that the recipient of the show cause notice may bring it to the notice. This helps in awarding the right kind of punishment.
Thanks,
Dinesh Divekar