When designing SVG files both of these tools are important. I use 2 simple rules when deciding if I need to unite and outline or create a compound path.
1. Is this a cursive font that needs to be united?
2. Is this a design that needs to be welded?
What is a compound path and how is it different than uniting an outline? Compound paths let you use 2 shapes to create one shape. (It's like using the welding tool in design space but you are able to export the final design.) If you notice in the image below I turned my type into an Outline (CTRL+SHIFT+O) and then used the "Unite" tool in the pathfinder panel on the top outline, and created a compound path (CTRL+8) or Object>Compound Path>Make on the bottom outline. If you go to the layers panel you'll see the unite only groups this outline or design because it's doesn't have any overlapping shapes instead of creating one shape.
Unite In Pathfinder
Unite allows you to group or combine 2 shapes into one shape but if they aren't touching or overlapping they won't combine into one shape they will only be grouped. Once a design is opened in Design Space the design will appear to be one shape but once won't cut properly. I use unite when I'm trying to make sure my cursive letters will export once turned into an outline. You can see in the example of the compound path below it created one shape, but the script font dashes aren't united.