Go with your needs

Suppose you own an awesome amp, something that gives you what you want to hear, do you still need pedal reinforcements?

I believe Marshall's JVM gives me what I want to hear even its mini-me version here. It doesn't need a BOSS DS-1 to please me. However, I still hook it up for two main reasons:

1. A different type of drive. That's right, the JVM on its own versus a JVM plus the DS-1 are two different voicings. I tend to tame the gain down on the JVM with the DS-1 in use. If the gain over at the amp is maxed out, the DS-1 presence wouldn't be heard. I just amplifies the background hissing more than anything else. I need a marked contribution from the DS-1 but it shouldn't overpower the amp. It's quite a balancing act depending on the guitars used. Most of the time it's also because I have different pickups in various guitars so there's no fixed setting that will adapt to the different guitars, it's the joy of tweaking (both amp & pedal) & discovering the sweet spot.
2. Increased sensitivity. That's right, I discovered that a cascading drive set up (doesn't matter if it's an amp-pedal or a pedal-pedal combo) gives off extra picking sensitivity. Simple reason- we are dealing with two sources of gain. It's definitely different from cranking the gain up on the amp only; that's only a single gain source. The set back is that increased background hissing.

So go with your needs. If you need to hear your favourite amp being kicked by a mild distortion unit so that you can become what you are (& not under-perform)- do it.