How Custom Written Software is a type of written software which is more useful than any other application software?

'Custom software' is not necessarily more useful than any other type of software, but it generally solves a problem that is specific to an particular organization or group.

OTS Software is short for Off-The-Shelf Software and applies to software that can be used as-is to solve a particular problem. For instance, if a word processor is needed this need can be filled by off-the-shelf solutions like MS Office or Open Office (free & open source). This software can be installed as-is and generally fits the needs of the average user.

Consider the scenario where the Off-the-shelf version does not quiet meet the needs of the user and needs slight modifications. This would be called MOTS software, short for Modified-Off-The-Shelf software and requires that a custom version of the OTS product is made by the original vendor or third party.

Often the problem is so unique to an organizations needs that there is no existing solution. In this case a completely custom software will need to be written. Examples might include booking software for a hotel chain, accounting software for a bank and so forth. There is not a wide user base for this type of software, so COTS and MOTS solutions are not available, or the organizations needs are so different that it makes more sense to build a solution from scratch.