Hi Tejas,
PLease be polite when you need some help from SCN forum, SCN forum is the source to get benefit when you are in need of something, Please try to elaborate your question further to get some inputs.
i hope you take this as positive.
however for your question answer is, when there is any new requirement from the client which is not available in standard system, than you have to go for custom one for ex: cheque format change, aging report, new forms, payment advices, new cost history report etc., than you will request the client to provide all the fields what they want, after you collect full details you will prepare Fucntional Spec to represent to ABAPer to develop custom table, report, t code etc.
i hope the above answer will help you to understand, also please search it in SCN abap development forum, you will see number of threads related to customized reports.
Regards,
Sree