"Always use a brim" is advice for a problem you might not have
Slicer defaults and forum advice often push toward always adding a brim, or worse, always adding a raft, as a blanket habit. For a large-footprint part in a stable material on a well-calibrated printer, that habit adds unnecessary material, print time, and trimming work for no real benefit. The useful version of this decision looks at your specific part's footprint, your material's warp tendency, your own history with first-layer problems, and how confident you are in your bed leveling, then picks the lightest aid that actually addresses the risk present.
How the decision works
A small footprint and a warp-prone material both push toward needing a brim; warp-proneness combined with genuine bed-level uncertainty pushes further toward a raft, since a raft is the most forgiving of an unlevel first layer. Prior first-layer issues add weight toward whichever aid the other factors already favor. Absent any of these signals, a plain skirt, priming the nozzle without adding real adhesion, is the default, not brim or raft.
Worked example
A 15 cm2 footprint, non-warp-prone material, no prior issues, confident bed leveling: none of the risk factors are present, so the tool recommends a plain skirt rather than defaulting to a brim out of habit, since nothing here suggests an adhesion problem is likely.
A common mistake
A frequent habit is leaving a large brim or full raft enabled by default long after the printer and bed surface have been dialed in well enough not to need it. Beyond the wasted material and trim time, an unnecessary brim on a part with a tight footprint tolerance can actually interfere with removal or leave a rough edge that then needs its own cleanup. Revisit this decision periodically, not just once.
Limitations
This chooses an adhesion aid from the footprint, warp risk, and history you describe, not a universal default recommended for every print. Your own printer's adhesion history with a specific material and bed surface, once you have it, is a better guide than any general rule.