I'll preface this my saying you will want to be cautious adjusting these too much cause I have read that its easy to get them to bind up. Also, they should be adjusted already to "level" the truck. You're probably fine to adjust a quarter inch or so but I would not be cranking these up super high...they're supposed to be a stock replacement shock. You will want to bump up to the 2.5s if you want a true coilover.
But, yeah you need to compress the spring when adjusting because of the way that you have to clamp the collar on the threads. The threads are so fine that if you leave the tension on the spring it's possible to not get fully engaged pressure on the threads which can cause the collar to get pushed down/off the shock body while you're driving.
In my experience the cheap compressors with the hooks don't really fit and you will need the type with the flat plates