Given sylenth1 as below

would there be a reason to use a third party delay for a synth sound of Sylenth1?

if so why?

 

Dont worry about it. "Use what you have" - and create some music

remember to PLAY and have FUN

 

As to my experience, yes. Simple reason, delays can in deed sound differently.
The more specific your idea of a delay sound is, the more delays you'll need to try.

 

can sylenth do multi-tap or convolution delay, if not theres your answer.

 

Another reason in this exaple of using the delay effect inside of a synth is the fact that it is going to be processed exactly like your synth on mixing stage, that's a thing you may not want everytime