May I ask what part of Datawatch Server you will be installing, as content is one of the 3 main components of Datawatch Server that can be implemented, the other 2 are Automator and RMS. Should you be installing one of the other 2 please do not mind the message and proceed with installing SQL Express 2014.
Hope this helps.
The Content edition of Datawatch Server (now known as Monarch Server) is a repository for securely storing, managing and controlling access to your assets - data prep models, documents, CSVs, etc.
This would be great if for instance you have a large number of sales reports and an accompanying model for those reports. You could set those up in the Content System and you'd be able to say, search for data based on index fields across all those reports - and from that result set, you could bring up the original report to see where that data came from. Very useful for governance.
Hope this gives you an idea.