A reactor makes sure the water flows thru the media. If placed in just a bag in a sump area you rely on water flow. An water always seeks the path of lease resistance.
I have the middle sized version - easy to setup, real easy to clean - a must.
It is a bit more expensive than say a Phosban reactor, but the easy of use and cleaning more than make up for it.
Media wise - it allows complete contact - vs a bag or mild placement.