The Joyo JF-38 Roll Boost is a great clean boost pedal. Whether you use it in front of the amp or in the effects loop is up to you. When it is used in the effects loop the Roll Boost is capable of giving you a massive volume boost, making it perfect as a solo boost.
When used in the effects loop it will give you a purely clean volume boost. If you use the Roll Boost in front of the amp but at the end of your effects chain then you will still get a volume boost but it won’t be as large and it will also add overdrive and sustain to your tone. If you use it before another pedal then it will overdrive that pedal more i:e: if you put it before an overdrive pedal then it will really boost the overdrive pedal giving you even more overdrive and harmonics.
You could also use the Roll Boost as a volume balancing pedal between two guitars where one is quieter than the other. For example, guitars with single coils are sometimes quieter than guitars with Humbuckers. You could set the roll boost so that when you turn it on it makes the single coil guitar the same volume as the humbucker guitar.